Your most logical option would be a Disk Data wipe using either DoD level 3-pass or NSA level 7-pass.

If you do not feel like doing a complete, fresh re-install of Windows then uninstall all applications you don't want them to use.

After backing up all your personal files, data, program settings, etc. that you might need, login as the actual admin account and under Windows Explorer delete the entire user profile (folder) under C:\Documents and Settings location (if it is Vista, then it should be at C:\Users).
Also CCleaner > Options > Settings > Secure Deletion with aforementioned settings would be good. Also on the main page of CCcleaner > Advanced > Wipe Free Space, then Options > Settings: select the "Wipe MFT Free Space"
for all applicable disk partitions.

If you get rid of your user profile and then do a secure data deletion, that should take care of it but here is another program that could ease the process.