File copy errors: Hard Drive, Controller, Drivers, or OS?

I get intermittent file corruption when copying files, e.g. I copied a directory of photos from one drive to another then did a binary comparison of the files and 5 - 10 % of the copied files differed from the source and had bit errors.

I have the latest bios and drivers, I’ve run chkdisk, reformatted drives, run Norton disk doctor and Checkit diagnostics and everything is reported healthy.

Here are my system specs:

Motherboard: Gigabyte GA-K8N51GMF-9
CPU: AMD Athlon 64 300+ S939 rev E
Memory: 2 x 512MB OCZ Tech DDR PC3200
Cooling: Stock CPU fan, case fan
Power supply: 400W
Hard Drives: 1 x Samsung SP0812C SATA 80GB (boot drive, 2 partitions c:\ & e:\), 2 x Hitachi HDS728080PLA380 SATA II 80 GB w/RAID 1 (1 partition f:\)
OS: Windows XP Home SP2

I’ve seen file corruption when copying in either direction between drives, when copying from other computers on my LAN, when downloading files from the web.

Is the problem with hard drives, controller, motherboard, drivers, os? Any suggestions for how to fix?

Thanks