|What make and model is your brand name computer, or if you have a generic desktop computer, what is the make and model of your mboard ?|
Using registry cleanup programs such as CCleaner to remove useless registry entries, and using Defrag, have very little if any noticeable effect regarding improving your computer performance on a modern computer.
Have you run (a) FULL scan(s) with anti-malware software to see if it(they) find anything ?
Windows 7 usually has Microsoft Security Essentials anti-malware software built into it.
Do you have any other anti-malware software program installed ?
Do you have more than one installed ?
If you DO, if it(they) has(have) (a) resident module(s) that run(s) all the time in the background looking for suspicious activity, if more than one of those is running at the same time, they are likely to CLASH with each other and cause problems, including probably slowing down Windows.
Some may even clash with Microsoft Security Essentials' resident module.
Many people install Malwarebytes, because it finds some types of malware most other major anti-malware software doesn't. (If you haven't tried it I recommend trying it). The frree version has no resident module, but it currently offers you a trial version of the full version when you use the free version. If you try the trial version, or if you buy the full version, if you also have AVG 2011 installed, I know from what I've seen while working on other people's computers that the resident modules of those too DO clash with each other - AVG 2011, particularly, cannot work properly when both of them are running, and the clashing does definately slow down Windows a lot.
"Task manager shows 75% use of physical memory"
That may or may not be relevant.
When you saw that figure, what was running ?
Only what was loaded when Windows was first loaded, or were you running something else yourself as well ?
(It will likely be higher when you are running something yourself.)
Was that right after Windows first loaded,or after, say, at least ten minutes ?
(It will likely be lower after some minutes, if you were not running anything in addition yourself.)
How much physical memory - installed in the mboard - is installed ?
(If you have less memory installed, the figure is likely to be higher.)
Is your Windows 7 32 bit or 64 bit ?
If it's 32 bit.....
The 4gb virtual memory address limit for 32 bit operating systems.
An example of 3gb working better than 4gb in a 32 bit operating system.
See Response 6:
Are you using onboard video - video built into the mboard - or video from a graphics card installed in a slot ?
(Your system will perform better with the same amount of installed ram when you do not use onboard video, most noticeable when a program benefits from the full bandwidth (max data transfer rate) of the ram being available - sharing the mboard's ram with onboard video as much as halves the available bandwidth.)
When you run Task Manager and look at the Processes list, is the % CPU usage at the bottom of the window high much of the time ?
It's normal for that to be higher for a while when you have first started up the computer, especially when it's the first time in a day, but that should diminish after a short while to a lot less when you're not doing anything yourself.
What size is the partition Windows is installed on ?
How much free space is left on it ?
If it has less than about 10% free space, that alone will cause Windows to run a lot slower than it is normally capable of .
"Is that why my broadband speeds are like watching paint dry?"
Maybe, maybe not.
" Videos play only for a few seconds before stalling for downloads to catch up."
Your max speed on the internet, whether online streaming of video or audio data works well, whether downloads have a higher or lower rate of data transfer, depends on many things, other than the performance of your computer otherwise.
See response 2 in this...
If you can download whatever video or other file that you were accessing on the internet and then run it on your computer, it it works flawlessly, or not flawlessly but a lot better, then you were probably having a problem related to your internet connection when you were trying to stream the same file's data on the internet..