You should start by backing up your data and other files which you need to keep. You should copy these files to somewhere you will not be formatting. (Another harddisk or CDs.)
Once you have done this, you should reinstall Windows. During the installation you will be asked if you wish to re-install the system you have, or whether you wish to erase your current system and install a fresh copy of Windows. As part of the latter option you have the option to format your harddisk.
So in short...make sure you have copied any files that you wish to keep and reinstall Windows, selecting the option to format your disk when prompted.
Dan Husin, December 2004