A quick read of some of the white papers on TDL-3 yields this:

"On a side note: the dropper won't infect the system if it runs in a
limited account or in an account with UAC activated."

http://www.prevx.com/blog/155/x-TDL-...follow-up.html

UAC is by no means bulletproof though, although setting it to its highest
level appears to help (what a pain). Microsoft backed off on the UAC
setting with Win7, defaulting the security level lower than it is in Vista.
And of course, this does nothing on XP installs. TDL-3 appears to have
been circulating since August of this year. There are two previous versions.