Help! Trying to re-load Windows 2000 Professional on my computer, but am having trouble.

Have two hard drives (C and D). Was running Windows from the C drive, but started getting an error "[some file] is missing or corrupted" and sometimes blue screens. Tried to reload Windows without success...somehow, the computer is now booting Windows from the D drive (I don't know why it started booting from the D Drive...the D drive was a drive from my old computer, so it does have Windows on it, but it is only intended to be a backup drive in the new computer...it is several years old and I don't trust it as a primary drive). So now, when I tried to reinstall Windows on the C Drive, when Windows does some install functions and then reboots...it boots up Windows from the D drive and does not finish the install on the C drive.

What I want to do: Stop the computer from booting from the D Drive so I can (hopefully) successfully re-load Windows on the C Drive and go back to using the C Drive as primary. Does anyone know how to (short of unplugging the D Drive) stop the computer from booting from the D Drive?

Thanks!!