I hate to start up a computer and have to wait "forever" (5 minutes or longer) for it to finish doing something so I can start using it! However, turning off Windows Update entirely isn't usually a good idea unless you have a routine where you check for them and install them yourself on a regular basis. I prefer to set the option to "check for updates and let me know they're available, but let me choose when to install them." If you use Microsoft Security Essentials then you can count on at least one update every day-- the latest antivirus database. Most of the other Windows updates seem to get released on Tuesdays.
Other than automatic updates, another thing that can tie up your computer for a lengthy period when you first turn it on are network devices (other computers, printers, etc.) that can't be reconnected to. When that happens you end up staring at a "dead" screen for several minutes when you turn on the computer-- it's hard to tell if it's working, because it won't even display a boot-up screen! When it finally does come up it will say it "couldn't reconnect all network devices" or some such. To prevent that, you can deselect the option to "reconnect automatically" when connecting to a network device. Or if you're using a laptop, boot it up and disconnect all network devices before you shut it down and take it somewhere away from your network.
