Stealth mbr virus need help Please

Hello:
New to group hope you can help.

First the system works fine except a glitch and boot up. (first clue)

the system boots and files run on win xp pro with all updates loaded.

anti virus programs trend pc cillion, norton, bit-defender show no virus present.

But Bit defender program getmbr gives error message no boot record, invaled partation table.

the only way to see the problem is to defrag. then you see the partation table not compressed, and files where ther not susposed to be.

This bug is in the wild.and a lot of users have it and do not know it.
I have 4 hard drives with the same problem. and cannot or not smart enough to be able to clean them.

I tried disk tool to zero drive, but i think virus went into memory, and went back on the drive.

If some can help with advice, please do.

Thank You...........Recon