To reduce your risk in the future, take the advice from the other guys, go to
http://www.zonelabs.com and get a firewall, it's free. Even without any programs running, NETBIOS (port 139) and IDENT (Port 113) will be open. You can go to
http://www.grc.com for a quick report on what is open (it only tests a few) or you can go to
http://www.dslreports.com for a more detailed scan.
http://www.hackerwhacker.com will test your system pretty hard, but you can only do one scan before you have to pay for the scans.
Trojans can get placed on your system quite easily, especially if you do a lot of downloading (from warez sites in particular).