Here's the problem...

When I'm minding my business on my computer, it sometimes freezes up, and my speaker makes a strange 'static' noise accompanied by a deep, beeping noise. After about 10-15 seconds, strange things happen.
It varies all the time...the computer shuts down, restarts, or the monitor suddenly turns blank and every 10 seconds, it turns blue, then blank again (the computer is still running during all this O_o).
Also, sometimes the computer locks up during startup (when the loading bar appears) and I have to reset the computer - the power button doesn't work.

This happens on the OS, Windows XP Home Edition SP3.

I have scanned it with AlYac v1.13, AhnLab V3 Internet Security 2007 Platinum and Avira AntiVir Personal.

Please help me out...

In Power Options do you have the sys set to restart if a failure occurs? Try setting it to No Restart. That is for any power problems. To prevent restarts resulting from software problems set your sys to not restart [uncheck the box] in Startup and Recovery options under System, Advanced tab. About the rest... try giving us a hijackthis log. Do this in order:
==Get CCleaner from http://www.ccleaner.com/ - and install it in a new folder. You should keep this one for general use. I set the installation checkboxes only to open from the recycle bin. It's neater that way.
Now run CCleaner from the recycle bin rclick menu using its default settings [if you set up CCleaner as i suggested, rclicking the bin icon should give you the Open CCleaner option...].
If you have FireFox open the Applications tab and ensure at least that Cookies and Cache are checked.
Select the Cleaner icon, press Run Cleaner.
Run CCleaner in any other Accounts.
==Please use IE or Firefox to do an online scan at panda:- http://www.pandasecurity.com/activescan/index/
-First Register [otherwise there will be no disinfection, merely detection] with a valid email address for the free online virus scan and follow through.
Unlike Kaspersky this scan does not require Java. Panda will clean only virii, but it is superb at listing other malwares which can then be targeted.
Please ATTACH to your post the log it produces.
==download hijackthis: http://www.majorgeeks.com/download5554.html
-copy it to a new FOLDER placed either alongside your program files or on your desktop and then... rename hijackthis.exe to imabunny.exe
-in that folder start HijackThis by dclicking the .exe; now close ALL other applications and any open windows including the explorer window containing HijackThis.
-click the Scan and Save a Logfile button. Post the log here.