System restore - has left old disk structure and I can't delete it!

I recently had to restore XP, and took the second option
on the restore disk, which makes an image of the old
system plus the user files which you can then copy back
into the new system once it has been restored. The
trouble is ,I cannot seem to get rid of the image - it's
20GB that I could do with! Anybody have any ideas how I
can do this?! [I have just come from XP, having been a
Win98 user for six years... ah, now that was an OS...]