I’ve had problems with trojans since the end of January. First, a.doginhispen and its spin-off b.skitodayplease showed up. They caused major problems for Internet Explorer. Norton did not pick them up. I got McAfee, and it picked them up, but they disabled crucial components of McAfee, and I was unable to remove them. While I was at work, hubby took a look and was able to get rid of them.
Then, a few days ago, trojan.advatrix showed up. McAfee did not pick it up. A full Symantec scan didn’t pick it up. However, a Symantec auto-scan kept picking up activity in the documents and settings folder, in a folder used by the Symantec program. A couple of tmp files were there; I looked at one, and found text in there (among a lot of gibberish) that indicated they were created by the trojan. After observing that new tmp files would appear upon computer start-up after deleting the previous ones, I figured the virus was hiding in that folder, and shift-deleted the folder so it wouldn’t go to the recycle bin. This trojan had also put some folders (Winbudget) in the program folder; I deleted those as well. On the Symantec website were instructions for editing the registry to remove entries made by that trojan, so I did that, too. It seems to have worked; I have not had the Symantec auto-scan window pop up again.
I have no idea how I got the trojans; when they appeared, I had downloaded nothing for several days. I rarely check email on this computer, and when I do, it’s to delete it, so they couldn’t have arrived as attachments.
There were a bunch of trojans that came in via Javascript links in web ads in late December and January. Just visiting a web site that hosted an infected ad was enough to get you infected. You didn't even have to click on the ad, it auto loaded the malware via Javascript when you visited the page. Disabling Javascript and blocking any and all ad links is the recommended course of action.