Well, it is an answerable question...
Backup your profile...
C:\Documents and settings\username\Favorites
......................................................................\Documents
......................................................................\Pictures
......................................................................\Desktop
Backup all Outlook PST files or contacts
Backup family pictures or mp3s
If you are a developer backup your files
Make a list of all the software you use on your computer and write it down to re-download or install later.
If you use FTP - backup the data file so you don't lose your sites
That will give you a good start on what to backup.