Sad thing is this post is the first result in google when I tried to search for it...
http://www.google.com/search?source=ig&amp;hl=en&amp;rlz=&amp;q=ut99+vista+server+search&amp;btnG=Google+Search

Apparently this was a problem in UT3 too and the patch supposedly fixed it - maybe you'll have to wait for a patch from Epic on the UT99 problem ;)