If you have a pc with a pre-installed Vista OS, there is usually a way to reset to factory defaults. How you do that depends on what make and type of pc you have, but I believe it was; Power on, and F12 straight away, (on an Advent laptop).
Do some searching for more info, and bear in mind you are likely to lose all of your work/music/etc. Save off the pc anything you value, to Flash sticks, DVD, or external HDD.