Before pressing 'Custom' or 'Express' buttons paste this text to the address bar and press enter:

javascript:void(window.g_sDisableWGACheck='all')

It turns off the trigger for the key check.

AV sez, "This week, Microsoft started requiring users to verifiy their serial number before using Windows Update. This effort to force users to either buy XP or tell them where you got the illegal copy is called 'Genuine Advantage.' It was cracked within 24 hours."