If you want to get a backslash it might be on a different key to start with, turn on the Number pad lock (or use an external keyboard), hold down the left Alt button and press 092 then release .
Otherwise from your command prompt do,
a:
format c:
Can I ask why you want to blank it? You could just boot of an OS CD, e.g. XP and then remove the partition that is there when you want to install it?
Steve