There are many many threads here on AO about
which is the best firewall?
So, please don't start this again...uhhh...bored... please use the search AO forums option next time.

But like hot_ice mentioned you should be ok with a good and properly working and configured firewall and a good updated virusscanner.

If you use an "always online" internet connection like xDSL or Cable then you could add some rules in your router if you use one or place a firewall box between your modem and home pc, other possibilitie is to use a hardware router/firewall/switch.
that could be from any firm but here are some examples,
from linksys: http://www.linksys.com/Products/group.asp?grid=23
or from Dlink:
http://www.dlink.com/products/DigitalHome/
http://www.dlink.com/products/DigitalHome/Broadband/
There are threads on AO about these products to:
search on DI-707, DI-704 or Dlink, or something similar...