A test by AV-Test.org found Windows Vista's pesky UAC feature does a very good job at keeping your Vista system rootkit-free.
The best of the all-purpose suites was Avira AntiVir Premium Security Suite, which found 29 active rootkits, with Norton finding as few as 18. The anti-rootkit tools fared better, with AVG Anti-Rootkit Free, GMER, Rootkit Unhooker LE, and Trend Micro Rootkit Buster achieving perfect scores. The scores for removal were patchy, however, with all failing to remove any of the rootkits they had found.
The results for Vista products were harder to assess because only six rootkits could run on the OS, but the testers had to turn off UAC to get even this far. Vista's UAC itself spotted everything thrown in front of it.
Only three of the 17 AV tools for Vista managed to both detect and successfully remove them, F-Secure Anti-Virus 2008, Panda Security Antivirus 2008, and Norton Antivirus 2008.
Once on a PC, rootkits can bury themselves quietly, but they have to get to that point first. As long as users interpret prompts from the UAC system attentively, or those messages haven't in some way been spoofed, rootkits struggle to jump to the PC without drawing attention to themselves.
That UAC can tell a user when a rootkit is trying to install itself is not in itself surprising, as Vista is supposedly engineered from the ground up to intercept all applications requests of any significance.