My computer won't boot.

My computer won't boot. It always prompts me to insert system disk. I made
it certain that there is no diskette in the floppy drive and my BIOS is
configured to make the computer boot from the hard drive. I have windows
xp, Compaq system (HP).



A little history. The computer had a problem before. It had similar
problem. But that time the black screen comes and asks to select one of the
followings: boot normally, boot in safe mode, boot form a net work. .. The
problem was a bad memory card. I replaced the memory card and hard drive
and installed windows ex from scratch, from Compaq disks. I installed all
the needed updates. The computer runs well for a while and then I started
to have an error message about file name something like "nwiz" (may be an
application file) the problem started. Some time I think the update my have
something to do with this problem.