What usernames are you refering to, windows usernames, ones in internet explorer, ones in aim? If it's IE you should be able to go into Tools -> Internet options -> Content -> Autocomplete and clear out the saved usernames/etc there and uncheck the autocomplete for username/passwords. Then go back to the main internet options screen and click on 'erase files'
And after that you should set Password protection for your system, starting with a BIOS password.
Use Windows XP and format your hard drive to NTFS file system, set user level password protection on sensitive folders and files.
When you've finished with the system in question, and no longer need to use it, format the hard drive and use a suitable utility to overwrite the entire hard drive, then format again and it'll be almost impossible for the information to ever be retrieved.
What the hell does anybody ever need that sort of secrecy for anyway?
This is in the wrong forum section, by the way - it should be in the relevent OS section or in the 'Security' section!