Generally the majority of virus checkers all do the main job of keeping out viruses assuming their definitions are up to date and this includes the free editions such as AVG Personal Ed., AntiVir personal edition and Avast Home Edition.
The personal free editions do lack many features of their big brothers and also are licensed only for home use on a single PC. A few free editions such as Avast requires free registration within the first 30 or 60 days to avail of continuous updates.
Generally the free checkers (including AVG) will check files placed on the PC, before opening, perform a scheduled scan and check incoming and outgoing E-mail. The drawback I find is that most do not automatically update their definitions unless the PC is left online overnight, although all of them can easily be updated manually (Double-click the AV tasktray icon and click on 'Update').
The pay virus checkers such as the professional versions tend to offer better features, give free technial support, check for other unwanted content such as Spyware and so on. Just beware that Norton charges around $30 for telephone technical support in the US.
In my experience, the AVG free edition I seems to have very little affect on system's performance and even when copying copies, but with the exception of when it is running a full scan. I seen McAfee and Norton cause performance issues on some PCs, especially when installing software or copying data. This is probably because the more sophisticated virus checkers do a lot more other checks.
Just bewarned that the majority virus checkers (whether free or pay) will not dodgy music (fake, adware infected, etc.) unless they contain an actual virus.