[Born to Conserve]

"Or buy a router. . . "

You need both software and a firewall. You can get the Linksys models that are not wireless for cheap on ebay. Everyone likes the wireless and is selling their old wired ones.

[TechJunkYard]

The router box would probably be fine. As long as it's set up to NOT forward aby ports back to the "victim" PC with the trojan listening to a specific port, there's no way the spammer could call it back.