CRC errors and file corruption

I have the following hardware:
EPOX EP-8RDA3+ mobo
AMD XP-1500 cpu
2 X 512MB non-parity non-ecc memory PC2100 sdrams
1 X Maxtor 40GB HDD
1 X Maxtor 80GB HDD
1 X ASUS DVD rom
1 X LG CD-RW

I am running windows XP sp1 and all latest patches.

I have the following problem:
When I download files I often get CRC errors or file corrption, I then re-download and sometimes it is okay. I have even had MD5 checksum discrepencies when coping a file from a DVD onto my HDD. I copy the same file from the same DVD onto another machine also running Windows XP and the MD5 checksum is correct.

I have tried doing scandisks and an load of ather diagnostic tool checks but found nothing. I even reformated my disks and re-installed XP with no joy.