I use AVG free edition to protect my personal computers and know of heaps of other people who use it and have never heard any bad reports.
I’ve found that it has protected my computers and I’ve not recieved any viruses whilst using it.
Ok, so trust me they have one, but seem to change where it is on their site every so often, it comes in handy when you want to do a scan with a quality product (I may cop some flack for that comment) but don’t want to pay for it.
Search for something like “Norton Security Scan” on their website and hopefully you’ll find it eventually.
Last time I saw it is was here: