I've been having rapidly degrading performance of my old tower, mostly random crashes and rough start-ups. I finally hit a spot where I was getting massive file corruption and I/O CD errors. I formatted with a fresh-from-the-shelf copy of Windows XP, but the install was rough. Many files would not copy (random files, not consistant files). I ended up losing the installation and had to buy another copy of WinXP (full install) which gave the same errors. Here's what's been changed since then.

New:
Antec Case + Power Supply
Asus P4P800S-X Motherboard
Maxtor 80gig HD
Sony CRX320E CD-RW/DVD
Intel P4 2.8a
PNY/Bravo 12 DDR (266)

That's basically a brand new box (sans vid card, Radeon 9600) and I have the Exact same symptoms. It took me a whole day to get a lucky install of WinXp that only missed 1 unimportant file. Most installations fail, and the ones that succed eventually fall prey to file corruption. This is across many discs, not just 1 particular set. Fresh retail, fresh burns, all discs have I/O errors and the HD corrupts any application that I run. For reference, it took a few tries to get a sucessful install of World of Warcraft. The next day a few critical files were corrupted.

Here's what I've tried:
Installing from the Hard-Drive
Making sure DMA is enabled on both IDEs
Defragment
Checkdisk
Installing from burned DVD
Cleaning CD media

With the same symptoms in TWO towers, I'm just at a loss as to what could be going on. Thanks for your time.

Barring any kind of weird problem with the vid card (highly unlikely but worth a check), I would first try using different IDE/SATA cables. But, seeing as how you replaced nearly everything, chances are you've already done that, if only by accident.

The next cultprit then is dirty line power. A surge protector is a great asset during a lightning storm, but it does jack $#^& for dirty electricity! See if an uninterruptible power supply solves your problems. APC makes some great units, and you wouldn't even need a really big unit. A common 300-500VA unit would be just fine.