For about a year I have been telling a story to highlight how users running as administrators are much more likely to get malware installed on their systems than users who run as normal users. The story is actually in Protect Your Windows Network if you wanted to see it. The conclusion was that if you let your users run as admins, prepare to spend a lot of time removing malware from their systems.
Recently eWeek did some empirical testing on that type of claim. Their results are presented at http://www.thechannelinsider.com/print_article2/0,1217,a=166172,00.asp. Basically, they verified the same thing we already know: no persistent malware showed up on the system where the user was not an administrator (note that I consider Power User privileges to be functionally equivalent to Administrators but eWeek separated the two).
It is really interesting reading. Take a look at it.