I don't know if these are compatible with Windows 7 yet but...
AVG is a reasonable antivirus and will scan emails. The interface is easy and the detection rate is fair. Don't have more than one antivirus as this can cause problems, but do use one that scans emails.
If your connection goes through a router you are protected from outside attacks, but you should use a firewall (not the Windows firewall) to alert you to any programs trying to communicate OUT from your pc. Zonealarm is effective and free and not bad for novice users.
Periodically, scan your computer with antispyware tools such as Spybot (very effective but not too intuitive) or Adaware (follow the links to the free version from the lavasoft website).
Finally, use Firefox to browse the internet instead of Internet Explorer, and also install SpywareBlaster from Javacool - all free, all very safe.
Then you have pretty solid protection from viruses and other nasties.