Well, I'm glad that the trojan has been identified. Now back to the original point of the thread.
If you have the Fasterfox extension installed on Firefox, and you have enabled the enhanced prefetching option, be cautious.
Fasterfox can, and did prefetch this trojan from the search results on Google, and I assume any of the other search engines. If avast! had not caught it, I would have been infected.
Following DavidR's lead (though his was for a different reason), I have disabled the enhanced prefetching. Firefox seems to run pretty much the same as with it on (with NoScript and Adblock Plus still installed), which seems to be much faster than Firefox alone.
As they say, "an ounce of prevention is worth a pound of cure." Though avast! caught it this time, no program has a 100% success rate, so thinking ahead might be a good thing.