I can fix it, but it's a little too involved to explain on a message board post. Try googling it. Pretty common problem.
Try this. Go to C:\Windows\SoftwareDistribution and delete ONLY the folder named "Download", then go
here and download the MS utility that fixes common Win update problems. Run it in default mode first, reboot, then see if that corrects the problem. If not, run it in the aggressive mode.
If it's Vista, there is a known issue with Win updates. Again, google it. There is a utility to correct it. Get it
here . Scroll down a bit and click the "Fixit" button