You have basically thee options if you want a free AV: AVG Free, Avast, and Avira.
Avira will NOT contain any trojans if you download it from a trusted source. People who claim that almost always either a pre-existing security issue and confuse the AV program alerting them to it after they install it, with the program having caused it, or they tried to get a "cracked" full copy that someone slipped a nasty surprise into.
What Avira DOES do, is pop up these annoying nag screens trying to get you to buy the paid version.
Avast is pretty good, but about once a year you have to go through a registration process. Minor hassle really, but a hassle none the less.
AVG Free is also pretty good, but it's gotten a bit bloated in recent versions. It IS the most comprehensive of the free virus scanners out there, doing some basic malware protection as well as virus/trojan/worm. I just haven't been a big fan of it since version 8 came out and they included this link scanner program. It's a good idea in theory... A small program is embedded into your browser that checks links on web pages to make sure that they don't contain some kind of malicious payload. Problem is it's been a HUGE headache for website managers, causing their bandwidth use to spike, and of course AVG has tried to make this look as much like normal browser traffic as possible. They have a legitimate reason for doing that... If it were easy to filter out, then ne'er do wells would simply do that. But I've noticed that my HTPC which uses AVG Free, is much slower loading web pages than my main desktop. The two systems are almost identical in specification, and the only major difference is my main desktop runs Kaspersky, and the HTPC uses AVG Free.
McAfee isn't even worth the price of free IMO. While it's detection rates seem pretty good overall, it will lop a good 25-33% off the top of your computer's performance, just as part of its normal operation. Norton is probably even worse, being the epitome of product bloat, combined with pretty regular false-positive issues.
I do like Kaspersky, and you can often find free after rebate, or nearly free, deals if you watch places like Fry's. If you don't live in SF Bay Area California, where you can barely throw a rock without hitting a Fry's, you can still usually get the deal on their website. Same goes for a number of other AV programs. Watch a site like Slickdeals long enough, and you're bound to see various deals involving AV programs.