Why o why does windows need to automatically restart after an update

there is a way to disable the windows update from automatically restarting your PC. here are the
ways:

For Windows XP Pro
go to Start, Run then type in gpedit.msc and press enter. then go to Computer Configuration, Administrative Template, Windows component, Windows Update, and find No auto-restart for scheduled Automatic Updates installations then double click on it. select Enabled. and thats it! close the window.

For Windows XP Home
go to Start, Run then type in regedit, and press Enter. then go to HKEY_LOCAL_MACHINE \Software\Policies \Microsoft\Windows \WindowsUpdate\AU. after that find this NoAutoRebootWithLoggedOnUsers value, by double clicking on it, and changing the new value to 1. thats it! close the window.

Note: there is also a way to stop Windows from keep asking you whether you want to restart your PC after an update. just go to Start, Run, then type in sc stop wuauserv. and press Enter. thats it! then after that just reboot according to your convinience.

OR, just disable automatic updates to totally disable the restart :P but unfortunately this way, you wouldnt get any prompt to update your Windows if theres any new Windows... aww....