|"The hard drive activity led is blinking on all the time when i have sound problems."|
That may indicate something is "pigging out" and using too much of the cpu activity time, which is interfering with the sound working properly.
If the hdd activity led is blinking or on steady all the time you're using the computer, that is NOT normal.
"The version of Windows Media Player that i have installed is Version 9.00.00.4510"
WMP 9 would have been sufficient.
The reason I asked what the WMP version is , is because WMP 10 and 11 have a feature "Share My Media" or similar that is disabled by default but if it's enabled I know that it bogs down Windows all the time despite the fact it doesn't use much CPU activity time.
However, sound problems when you're playing something on the web can be caused solely by a poor internet connection.
Have you tried downloading any of the YouTube videos, then playing them?
"i have a modem at home but i use wireless connection because"
What were you going to say after that ?
Is this a laptop and you use it both at home and at other places ?
If so, do you have the same sound problems wherever you connect to the internet ?
You can have all sorts of problems when you connect wirelessly, especially if you're using a wireless G or B connection rather than a wireless N connection (both ends of the wireless connection have to support wireless N). Usually the problems are directly related to the wireless signal connection between your computer and whatever you're connected to not having enough signal strength, as detected by your computer.
In general, you're a lot more likely to have problems with the reliability of the internet connection when you are connecting through a DSL or ADSL high speed modem rather than a cable high speed modem.
"The C partition(HP_RECOVERY (C:)) is 7,32 GB and free space left on it is 6,08 GB.The local disc (D:)is 48,5 GB and free space left on it 32,6 GB,so i don't think the problem is the memory"
Obviously you have re-loaded Windows, or you have loaded a second installation of Windows on the HP RECOVERY partition, at some point. The HP RECOVERY partition would be assigned D, not C, on the original HP software installation.
You have plenty of free space on both partitions, so not having enough free space left on the partition Windows is running from is not your problem.
"i hope that it's not virus,malware or some s--- like that!!!!!"
Have you done a Full scan of your computer with whatever anti-malware software you have installed ?
If NO,do that !
If YES, or in any case, you could try downloading, installing the free version of Malwarebytes, and running a Full scan of both C and D with it . It finds and gets rid of a lot of a lot of crap other anti-malware can't even find. It takes less time to do a full scan when you run it in Safe mode.
".but...did viruses or malwares exist on links?????i thought that viruses and etc. are only on programs you download."
Most malware these days is not a virus - it's something else.
There are web sites that are contaminated with malware crap and all you need to do is click on a link to that web site and the crap will load onto your computer. Often such sites show you graphics that have nothing to do with the title or description of what you clicked on.
You are more likely to get malware when you visit web sites that have software downloads that have been illegally obtained (movies, music, software, videos, etc.) and if you download from those sites sometimes there is malware embedded in the download that is released when you open up the compressed archive file or play whatever it is.
Scan your computer for malware first.
If you can do that in Safe mode with whatever anti-malware program it is,it wil ltake a lot less time to full a full scan.
The free or paid version of Malwarebytes, the free or paid version of AVG 2011 will run in Safe mode
Microsoft's Windows Defender, Microsoft Security Essentials, McAfee software, will not run in Safe mode.
(Press the F8 key repeatedly while booting, when the boot choices menu appears, choose Safe mode, or Safe mode with networking.)
If that doesn't help there are other troubleshooting things we can suggest.