If you want to tell your friends about the do's and dont's in your computer when they login in your absence. You can do it easily by displaying a legal notice at system start up.
REGEDIT [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system] "legalnoticecaption"="enter your notice caption"
"legalnoticetext"="enter your legal notice text"
Automatic Administrator Login: Here's the trick which you can use to prove that Windows XP is not at all secure as multi-user operating system. Hacking the system registry from any account having access to system registry puts you in to the administrator account.
REGEDIT 4 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon] "AutoAdminLogon"="1"
No Shutdown:
If you want to play with your friends by removing the shutdown option from start menu in their computer.
Regedit HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer "NoClose"="DWORD:1"
