I second what Dinesh said. There are a number of free antivirus solutions available.
Also, if you're using the Windows 7 firewall, look into adding the firewall controller - Windows 7 Firewall Control : Sphinx Software
- the free one will help you control outbound connections without having to tweak the firewall policies themselves.
(A slightly more expensive solution would be to buy your own router, a bit of overkill for a single system, but it may prove to be a good investment if you get extra gear, like a networked printer or network storage drive or want to play LAN games with friends.)