If you are worried you might already have a virus, download Malwarebytes and run it, and also downloading and running Stinger. You can (or might have to) run them from Safe Mode if you have a particularly nasty virus onboard.
Personally, once I cleaned up our fiasco of a machine with those two pieces of software (we had a foul piece of malware on it), I started running AVG and haven't had any problems since. My BIL, who is in IT, highly recommended it when I asked what antivirus software he runs.
Good luck, and if you find you have trouble getting antivirus software to run, it's sometimes a strong indicator that your PC is already infected with something (ie: can't install/download/activate your AV software), which is why I mentioned the first stuff.
ETA: We were running McAfee, paid version, when we had our issue with trojans...FWIW. I had it set to update virus defs automatically, but clearly it didn't pick up on some trojan...ended up not being able to open an IE window and getting funky pop-ups. Cleaned up the machine as described above, and no problems since.