[Ham-Computers] BIG Question

[Gene WØQFC]

I just run into something that has me puzzled.
I have a computer that was dead - would not boot - and found that the PS was
shot.
Replace the PS and still no boot.
Replaced the CPU and still no boot.
Replace the MOBO (with new CPU) and still no boot.
Replace the HDD and it now boots.
(I worked backwards and used the original MOBO & CPU, but they were BOTH
shot).
DUH!!!!!!
Anyway, after I installed WIN XP on a replacement HDD and got the machine
going again, I hooked up the original HDD as a slave and guess what? It
would not boot!
It would go through the POST and then keep re-booting itself. Take out the
Original HDD and viola! - it works.
Sooooooo, the big question is: could a virus cause the problem with the
machine not booting? 
(I think that the PS going west was something would have happened anyway,
but not sure if it had anything to do with sending the MOBO & CPU out West).
I would not worry about it but there is stuff on the original HDD that needs
to be recovered....
Any ideas out there????

Gene, WØQFC