A virus has to be loaded in to your PC from an external source i.e. Interent/disc etc. My advice would be to not to open E-mails if you do not know thier source. You can virus check any disk you want to load using Norton etc.
I also recommend to install The Cleaner (www.moosoft.com) as this program protects you much better against Trojans. Anti-virus programs, unfortunately, do not detect all Trojans. If you have a broadband connection, such as cable or DSL, I also recommend to install a firewall (www.zonealarm.com). Freeware.
Cheers,
PD