Here's what I'd do (and have done in the past)
Boot in to Safe Mode, log in using the built in system administrator account, Go to Control Panel / User Accounts, delete the personal user accounts, delete their data/files when given the option during deletion of user account.
Reboot
It should now login using the built in System Admin account in Windows normal mode.
Download and install
Eraser in readinesss for use.
Got to
C:\Documents and Settings and delete any left over user account folders {that are sometimes left over}
Now Right click on your C drive and select
Erase Unused Space. Click on
'Options' button and highlight option
4 - Pseudorandom data, click on the
EDIT button, change the number of passes to
3. OK
Ensure all 3 overwrite options are ticked on main window.
Click OK to begin secure wipe of free disk space
Once done, shut down, job done