Just follow a few simple rules and you`ll be fine

1) Install a good Virus scanner(Norton, InoculateIT, etc) and keep it up to date!!
2) NEVER run executable files that come from a source you don`t trust, and even if you do scan them first.
3) Install a firewall (ZoneAlarm, Tiny are good)
4) If you use Express make sure all unsafe content is disabled and be careful with email address you do not know.

Anyone add some more?