My maxtor did this middle of last week. Fortunately it was part of a raid 1 mirror and my advance rma will be here on Friday.
Maxtor is slow, took them till today to get it shipped.
Seeing as I'm running only one hard drive which is a wd rma replacement I made a backup.
I'll give you a list of things I backed up. May help you in figuring out what to back up.
Documents and settings stuff - Outlook .pst file (mail), favorites, cookies, program settings(e.g. ftp program connection list)
Program Files - Game saves, icq & aim received files, winamp skins, anything else that saves stuff in it's folder.
All media (pictures, audio, movies)
Anything else you've created
Windows registry. This isn't totally necessary but it can come in handy if you don't want to redo the settings for some program.
If I forgot anything anyone, feel free to add to the list.