system wont recognize hard drive

[alockshin]

we removed a hard drive from a HP pavillion put it on a second computer and removed 17 viruses.
we then put the drive back in the original machine and we can only get an 'a' prompt. the bios shows the hard drive is installed, but there is no way to access it.
we do this proceedure 10-15 times per day on other computers, and have never seen this problem. any suggestions?
please email me at alockshin@houston.rr.com
thank you.

a.l.

[degsy]

use a startup disk and check that the boot files are still on the C drive.

If not use the SYS command to transfer the boot files.