To make the username clear on log in or logoff (do this)
1. Start-Programs-Administrative Tools-Local Security Settings
2. Click, "Local Policy"
3. Click, "Security Options"
4. Scroll down to "Do not display last user name in logon screen" and Double Click
5. Check, "Enable"
6. Click "OK"

Thats it
If this is XP Home - Don't expect much to any security. It is meant for simplicity not security (there is always a tradeoff).

-Sparky