When downloading SP1 and later reinstalling Vista I never had any problems seeing the service pack go on like some with Dell, HP, Gateway prebuilds have run into. But the main thing is seeing the problem there cleared up.
One thing some people run into when going to download direct from MS is clicking on the IT developer's placed right on top of the three links. The second mirror is for the average user. Now figure that one out why MS set it up that way?
If you grab the wrong one it can goof things up.