I have had this problem occasionally on my Vista Home Basic but not on Home Premium. It sometimes does take quite a while to get under way so it is important to be patient, even waiting 15-20 minutes to see if it will take off. If I hard shutdown and then restart, it usually fixes itself. Again, patience is important. When you are used to XP just getting cracking and doing its thing, Vista can be infuriatingly slow at routine tasks at times.
Sometimes there is a conflict with some stuff partly downloaded and emptying a folder called SoftwareDistribution in C:Windows will enable things to start again and complete successfully.
If this does not work for you, MS offers free phone help for update problems.
Good luck. Let us know how it goes for you.
Edited by rowal5555, 18 September 2008 - 02:43 AM.