There are a number of excellent free AV utilities, including AVG7 and Avast.
I used to use AVG but switched to Avast after installing it on a computer newbies's system recently because it updates automatically.
You can download and install it from: click here
When you have installed it, make sure you obtain the proper key rather than the temporary one, as it suffices for more than a year before having to be renewed (free in both cases).
It's explained on the website.