Barring some situation where you've got incriminating stuff on the drive, your best approach to this problem is just to be honest. Approach the IT department and state the following case:
-I've been here for ten years. While I didn't squander my time on personal crap, there's bound to be a personal email or credit card number somewhere in a browser cache or temp file.
-I've got (X instant message/skype/whatever) account running on login, and I don't want to leave that hanging around.
-What can we do to protect both myself and the company?
Suggest a drive wipe and reimage. It's a bit overkill, so if that's an issue suggest you be given supervised admin privileges to wipe temp folders, uninstall software, wipe leftover program and document folders, delete your old user account, etc. Ask about a place to back up all your work files in case anyone needs them.
Of course, your IT might not be retarded, but it doesn't hurt to ask. And if you've got admin privileges, do all that stuff yourself.