Of course all is a matter of personal opinion on most things, however Avast rates higher generally on protection than AVG and also in speed. Also consider Avira Free. Excellent, doesn't cause slowdowns either. I have used it myself for several years and am quite pleased.
You might check out reviews HERE
For a more then a year I am suggesting to friedns the Comodo suite. It is free and from some tests it shows as "excellent".
If you only need the antivirus portion, then you can disable to install the firewall portion.
Anyway, I would suggest Avast from those two.
But basically a day after the leading Antivurs vendors find "something" or do an update, then all vendors update their databases for their software to fight the "something".
Any opinions between AVG free version and Avast free version? Any experience with either of them slowing up the computer? Thanks.
Just in case google for some comparisons of Antivirus applications. It is like with security, there is never a 100% security. You'll get a different answer from any IT person. If the IT person came accross a any problem, then (s)he will tell you to get the other one.
The main aspect is here, that the AV shouldn't yell on any false alarm and at the same deal with all possitive alarm(infection). You can greatly decrease any infection by safely browsing on net, using know applications and so on. And if you need to test an "unknown applications" for any reasons that I don't want to know, create a virtual OS and run it there. Or event browse in the Virtual OS on "dangerous" websites..
Anyway you should once a time download a kind of portable antivirus from a different vendors as you have and run it.