As a few folks have said already AVG or Avast free editions are both very good anti virus programmes. If ever I build a machine or look after one for a friend, the first thing I do is put a range of software on to try and keep them safe online.
There is also Comodo, but as it also has a firewall included it can be a bit of a pain at first when it alerts at everything until it has learnt what software you're using.
If you're using windows Microsoft have recently adde their Security Essentials package, which is getting some good reviews, but it doesn't have a firewall
http://www.microsoft.com/Security_Essentials/
You can find all of the above either on the home page of the company or from Filehippo (all downloads from filehippo are free from viruses and malware/spyware) who also have a range of other anti spyware/malware products and other useful software.
http://www.filehippo.com/
Filehippo also has an update checker programme, which will tell you if any version of your software such as Adobe Reader or Flash are out of date. Some nasties exploit weaknesses in older software versions so it's best to try and keep everything up to date if you can.
Hope this helps, if I can help any further please pop a post on here
Jo.
2012 Fundraising for the National Rheumatoid Arthritis Society (NRAS)
http://uk.virginmoneygiving.com/RoboJo