I found with Vista install, and Vista SP1 - nothing would work as long as I had in excess of 3GB RAM.
I had to take out three sticks (at 2gb each) to get below the 3GB maximum, do the install/update then put them all back in again.
It even taught Microsoft in Mumbai - they didn't know that it wouldn't work above 3GB either.
I haven't attempted SP2 for the same reason.