Now days a lot of computers are comming with a partition containing restore
information. The very first screen you see when you startup will usually tell
you to press one of the F keys to get to the restore utility. There may also
be other ways of getting to this utility from within windows depending on
your manufacturer.
If this is not the case, you should have received CDs of all of the software
that came loaded on your computer. Follow the steps listed in the previous
response (re-install windows) and then install any additional applications
that came on CD.