A quick way to stop Windows Update from asking you to reboot is to simply:

1. Go to Start, Run
2. Type ‘net stop wuauserv’, and click OK

This will stop the service. I don’t see this as a problem, as the Windows Update client isn’t going to download any more updates while waiting for a reboot anyway. Once you reboot, the service will start again.

Obviously this will only work if you’ve got the correct privileges to stop and start services in Windows.