Computer slow overnight, nothing seems to solve

Well, very weird problem here, disturbing and frustrating too.

Somewhere around December 20, 2010 I started the computer as normal, the night before that morning it just worked perfectly, but suddenly while I try to use it, I noticed it became a lot slower.
It's January 5, 2011 now, and I have been trying stuff almost non-stop, I have scanned for viruses, malware, defragmentated my hard drive, all in normal as well as in safe mode. I even reinstalled Windows XP multiple times, nothing helps.

I can do some things without being interrupted by 'lag', most of the time, I even tried gaming which works OK at random moments.
I am thinking it is a hardware problem, I even took out my videocard and ran the comp just on my old onboard videocard but that didn't solve it.
I even cleaned the whole computer, took out all the dust etc. nothing seems to work.

I am beginning to think it's a damaged CPU or something, while I'm a pretty advanced user and spent alot of time on the computer, this is unsolvable on my own.

It worked perfectly for years...

My computer specs:

Nvidia Geforce GTX 260
Intel Core 2 Duo 4400 2.00ghz
Windows XP Professional (with SP 3)
Zalman ZM600-HP


The computer is just a Packard Bell Imedia J9250 with a custom power supply, videocard, sound card and added RAM if I remember right (it all worked fine before)

Thanks in advance.