Since April First is just around the corner, why not have some fun with your friend's computer:
1. Right-click the Start menu.
2. Select Explore Users.
3. Navigate to Programs > Startup.
4. Right-click the file windows and select New > Shortcut.
5. When the window pops up asking for the location, enter
This will cause his computer to shutdown right after booting up.
Start up in safemode (or hold down the shift key during startup) to disable it. Then remove the shortcut.
1. Right-click the Start menu.
2. Select Explore Users.
3. Navigate to Programs > Startup.
4. Right-click the file windows and select New > Shortcut.
5. When the window pops up asking for the location, enter
"%windir%system32shutdown.exe -r -t 00."
This will cause his computer to shutdown right after booting up.

Start up in safemode (or hold down the shift key during startup) to disable it. Then remove the shortcut.