uac causing explorer exe use 100 cpu

I have installed vista 64 bit and upon first startup and before installing anything I found that my cpu usage was between 90 and 100%. I tracked around and found that explorer.exe was the cpu hog. At first i try'd disabling each service in turn but this didn't seem to help. i started up in safe made and all was fine and cpu useage was only 1 to 4% at idle. i booted back into normal mode and was back to 100% cpu. I disabled UAC and bingo cpu usage fell straight away to normal levels. has anybody had this problem or know of a fix. My 32bit vista does not have this problem. I try'd downloading the 64bit again and each time used the microsoft download manager

Sorry , i didn't double post intentionally, i got errors saying it was unable to post , so i try'd again.