Computer help: PC Freezes Constantly

PC Freezes Constantly

I posted this in the Dell forum, but thought I would try here too.

I have a Dell Dimension E521, bought in Sept. 2006, which has worked beautifully until this week. All of a sudden, out of the blue, it's freezing up every time I try to use it. No matter what program I use, it will freeze. Sometimes I reboot and it freezes again. Sometimes I reboot and it freezes 10 minutes later. Ctrl-Alt-Del doesn't work - nothing works. I have to push the power button. It's taken me 3 tries just to write this post.

I first tried System Restore. Didn't work. I have McAfee Security Center and I ran a full scan - nothing. I checked Disk Defrag - my files are fine, no need to defrag. I ran the Error Checking tool on the C drive - no errors. I ran CCleaner - nothing unusual. I ran Ad-Aware - found a few random cookies, which it deleted, but still freezing up. I ran Malwarebytes' Anti-Malware - nothing.

I put in my Windows XP disc that came with the PC and tried to Repair Windows - but there was nothing to repair. I opened up the actual PC case to check for loose components and/or dust - everything is rock solid and shockingly, there was no dust.

I haven't installed any new programs in 4-5 months and I haven't downloaded anything from the Internet. When ever I've had any glitches in the past, everything I tried above has always worked. I give up, so here I am. Any ideas besides FDisk'ing the whole thing or buying a new PC? Below are my specs:

Dell Dimension E521
Windows XP SP3
AMD Athlon 62 X2 4200+
2GB DDR2 RAM (4x512)
256MB nVidia GeForce 8600 GT
250GB Hard Drive
DVD-Rom & DVD/CD-RW