Reformatting - a complete reinstall of the PC software. The hard disk is wiped clean and then you reinstall windows and all it's software.
Before you reformat, make sure you have backups of all the data (but then you have that already dont you
) as you will want to copy that back on, and the installation disks and settings for windows, all the add on hardware drivers, togehter with other programs you have used such as the antivirus, wordprocessor, games, etc.