UAC Prompt Very slow to display


I have been having this problem for a number of weeks, and it started
about the second day after installing Vista. My UAC prompt takes between
10 and 15 seconds to display. I have turned off the change to secure
desktop, but it made no difference.

I have also noticed (I do not know if this is connected) that when I
want to preview an image it takes the same amount of time for the viewer
to appear.

Apart from these two niggles, my machine runs great, and is really

Anybody have any ideas on what may be causing this?