OK .......... running an anti-virus programme is normally a good idea.
You never know what little bugs and viruses will get sent to you .
Here is AVG... its free and updates automatically with new information on viruses.
Then click on downloads.
Any anti-virus is only as good as it's last update, so whilst it's always advisable to have a real-time virus checker like these running all the time, I also run Housecall once a week because it updates weekly.