This may work on Vista but i haven't tried it yet.
Here's what you do, you need to be an admin to create an account or use the admin tools. You need to have an account ready, this will be the invisible account. you can add a new account or use an existing one
1. Go to Run and type "regedit"
2. Press the plus by "HKEY_LOCAL_MACHINE"
3. Then the plus by "SOFTWARE"
4. Then "Microsoft"
5. And then go to "WindowsNT"
6. Go to "CurrentVersion"
7. Then "Winlogon"
8. Then "SpecialAccounts"
9. And finally just click on "UserList"
don't press the plus
10. Right click and create a "DWORD" value
11. Put the value name as the hidden user account name
12. And set the "Value Data" to 0
13. Exit the window then reboot.
NOTE: To make this less suspicious use the windows welcome screen instead of the plain text one. The welcome screen shows all user accounts except the hidden one. Now to login from the welcome screen, hold CTRL+ALT and double tap DELETE. This will show the plain text version of the welcome screen that does not display user accounts. Simply put in the hidden user account's user name and password and you're in.
As I said before I have not tested this in vista.
And if you need me to i will post screenshots