Two things:
Is the 60Gb drive set up with one partition (just the C: drive) or is there maybe 2 partitions (C: and D: for example). I know Dell sometimes do this.
Secondly, in XP there is a Search option on the Start menu (I assume Vista has something similar) and you can search on your hard disk for objects by size.
Search your disk and see if there are any large files you dont know about. Maybe your DVD burning has left some large files in a temporary folder for example.