|It wasn't clear what you meant by "the RAM showed that it's using a little more than 3 GB."|
Since there was no indication in your posts that you have the 64 bit version, I assumed you had a 32 bit operating system and the usual thing - you noticed Windows was not using all of the 4gb of ram.
(All references are regarding what I see in Vista Home Premium, with SP2 updates installed - they're probably the same in any version of Windows 7, except where noted.)
If you look in System Information (shortcut - type: msinfo32 in the Start Search box above the Windows 7 "Start" icon, press Enter)
on the right hand side
Your Installed Physical Memory amount is the amount of ram installed in the mboard. 4gb in your case.
Your Total Physical Memory amount is the amount that is available for Windows itself. When you are NOT using onboard video, that's the same as the amount of ram installed on the mboard. When you ARE (or the mboard is, whether you're aware it is or not) using onboard video, that's the amount installed in the mboard minus the amount of that that's shared with the onboard video.
NOTE that some new and more recent mboards that have onboard video have a Hybrid video feature. When you install a video card in a PCI-E card slot on such a mboard, the onboard video may NOT be disabled automatically, and in that case, the Total Physical Memory amount will be less than the Installed Physical Memory amount , the same value it is when you are using only the onboard video.
Also NOTE that some mboard bioses subtract the amount of ram shared with onboard video if it's being used from the total ram installed in the mboard, and in that case, that may be what's reported as the Installed Physical Memory in Windows (that's certainly the case for XP and 2000 - they have only the Total Physical Memory amount in System Information). However, that probably is not the case for new or fairly recent mboards.
The Available Physical Memory is the amount that is presently free to be used by Windows. That varies dynamically depending on what programs are running in Windows at the time you look in System Information.
The Total Virtual Memory Amount is a combination of the amount of physical ram that is available to Windows (minus the amount shared with onboard video if that applies) and the size of the Windows swap (Page) file on the hard drive partition Windows is running from, at the time you looked in System Information. Generally, the more ram that is installed on the mboard, the smaller the swap (Page) file on the hard drive is, but by default Windows dynamically changes the size of the swap (Page) file depending on what it deems is needed at the time.
The Available Virtual Memory amount is the amount of it available after the amounts needed for programs presently running have been deducted, at the time you looked in System Information.
Page File Space - the size of the Windows swap (Page) file on the hard drive partition Windows is running from, at the time you looked in System Information. It changes dynamically.
When you look in Task Manager, under Processes, it shows you how much memory is designated to be used by programs. That amount may change dynamically when the program is actually using CPU time. If a program is not using CPU time, then that program has no effect on the computer's performance, other than the amount of memory that is designated to be used by it is "reserved".
If you click on CPU twice, the programs are listed with the ones that are using the most CPU time at the top, in descending order, and that list is dynamically updated about twice a second.
"...the CPU wasn't stressed or anything, cause it was using about 10%-20%..."
The total CPU percentage presently being used doesn't necessarily directly indicate the impact that has on the performance of the computer. There are some prgrams that will bog down everything in Windows depite the fact that they don't use a lot of cpu time. Of course, if the CPU percentage is high or 100%, the performnce is going to be affected for the worse, but you can't assume that for lesser percentages.
"no malware or antivirus program was running..."
I assume you meant
no ANTI-malware or antivirus program was running
By anti-malware software I mean anti-virus, anti-spyware, anti-trojan, anti-rootkit, etc., etc., prgrams.
Most malware these days is NOT a virus - it's something else.
Most if not all "anti-virus" programs have modules that detect other malware as well.
You may have no full anti-malware scan going on in the background, but the chances are high that you DO have at least one resident anti-malware module that is running all the time.
Many anti-malware programs have at least one resident module - a part that runs all the time looking for suspicious activity - it's CPU activity varies dynamically, but it's running all the time.
Some anti-malware programs also have an obvious firewall module (e.g. paid AVG 2011) or a firewall like module (e.g. free or paid Spybot, paid AdAware ) that runs all the time - it's CPU activity varies dynamically.
I have discovered at least some Vista installations have Microsoft's Windows Defender anti-malware program built into the operating system. (My Vista installation doesn't have it - it is Vista with no SP updates updated with SP1 and SP2 updates. A friend has it built into Vista Home Premium on a HP computer that was probably originally Vista with SP1 updates included. )
In that case, it's listed in the running Processes in Task Manager, but it is not listed in Control Panel - Programs and Features.
I don't know whether that applies to Windows 7 or not.
Windows Defender clashes with some anti-malware programs. If it IS listed in the running Processes in Task Manager, but it is NOT listed in Control Panel - Programs and Features, you can't un-install it, but you can stop it from running in settings in the program itself.
Unlike most anti-malware programs, apparently you can't run Windows Defender in Safe mode, or un-install it in Safe mode.
If you don't know whether your anti-malware program(s) has (have) (a) resident module(s), tell us what programs are installed on your computer.
"my HDD is a 7200 RPM SATA 6gb/s,"
Some hard drive specs are hype. There is no such thing as a conventional hard drive that has a max burst data transfer speed faster than 300mb/sec (the situation is different for solid state hard drives, but very few people are using them)
Your hard drive is NOT transferring data at it's max burst data transfer speed all the time - that is only used when it's needed, and when it is needed it's only used for brief periods of time in one go. Most of the time your hard drive is not actually running anywhere near as fast as that, and in some situations, when you are moving huge amounts of data, the max it can run at is it's sustained (continuous) data transfer rate, and no hard drive I know of has a sustained (continuous) data transfer rate higher than 100mb/sec.
"It only lag when I extract a movie from .rar files or joining the movie parts together with File Splitter & Joiner. And I don't know whether you would call those operations file transferring or not, but aside from that, my PC is normal and pretty fast."
"....aside from that, my PC is normal and pretty fast."
In that case, it may be annoying to you, but it isn't a significant problem.
If you had said that in your first post, I probably would NOT have bothered to make any answer post, and even if I did, I certainly would not wasted my time mentioning most of the stuff I did in this subject.
Extracting a movie with WinRar or whatever impacts the system more that extracting most other things, because the original archive is already nearly as small as it can be before it's extracted, it's HUGE, and it's often a more complicated procedure to extract the file or files in it.
"....joining the movie parts together with File Splitter & Joiner..."
You didn't mention that previously. That's likely to impact the performance of your system a lot more than just extracting the movie.