Do a manual update (click start > all programs it should be near the top), it's probably best if you can do this at a time you can start it going and then just leave it alone until it's finished.
Windows tries to update itself quietly in the background by default, the problem with this is it will download the updates very slowly if you are using the computer as your use takes priority over the updates, if you normally have your computer switched off and only turn it on when you use it, then turn it straight off again it tends to never get time to finish the updates.