Along the lines of what jholland said:

~ We need to know the system specs (Operating System with applicable service packs, etc)

~ Was there anything installed or unsintalled (software and hardware wise) prior to the issue that might have something to do with the problem?

~ At what point does the error come up?
a) Before the BIOS screen (unlikely)?
b) After the BIOS screen but before Windows starts loading (likely point)?
c) After Windows partially loaded (logo is displayed once or twice) but before getting to the desktop (possible but not very common)?

~TL
PS. I am guessing this is a Windows 9x or ME system?