MCB Chain Corrupt

[bullgator]

I am getting this error which relates to low level system memory and possibly a memory resident recursive virus. Any clues ?
This is on a Toshiba Tecra 8000 P2-400, 256mgs, 6gig.
I bought it with XP Pro installed, and have since:
Wiped hard drive, installed with W2k, Wiped againand installed 98se, wiped again installed dos, and have tried virus scans on each...nothing. When I try to run antivirus from a boot diagnostic cd I get the error: Panic: mcb chain corrupt.
The end result is that in any of the os's I get strange keyboard input. Any thoughts on how to scan memory for virus ?

Thanks

[GeorgesPC]

Have you tried MemTest86? http://www.memtest86.com/

[bullgator]

I haven't been able to get it to run from ultimate boot cd, but followed your link and was able to get the iso. It's running now
Thanks !
(there is no floppy on this machine)