Random CRC errors in file archives

Lately I’ve been getting CRC errors when extracting large setup files (such as the 3DMark 03 demo download) or large multipart archive files (where it is usually different files which throw up the CRC error each time I try to extract them). I read elsewhere that it may be a RAM problem, so I tried extracting/installing with only one of my three PC133 SIMMs in at a time, still with the same problem. Another possibility was that it might be an HDD I/O error, so I replaced the Hard Drive, still with the same problem. Any more ideas or things to try would be greatly appreciated as I can’t trust anything I download to work properley! Thanks

System specs:

AMD Athlon XP 1800+
448 RAM
Abit KT7A mobo
Geforce 440 MX
Sound Blaster Live Player 1024
Pinnacle PCTV Rave TV Tuner card