Hi did,
So sorry to hear about your son.
the main thing to backup is C:\documents and settings
This contains all users "my documents" folders and the desktops. it also contains your favorites for IE and most of the other user specific files.
You do not want to backup program files or the wondows directory. but check what else is on the C drive other than the above 3 folders. if there is anything else then it is a maybe. either post the folder names and somebody will let your know or if you have enough space back them up anyhow. it's better to backup something you don't need than to not backup something you do need.