If the person you had the argument with about UT ( i hope the OG version 99 and instagib :P) was also the server admin or in control of the server then he or she may have given you all your 'presents'. There is an ammount of trust by playing on a UT server since the te game will install maps packages or whatever it needs to play, A shady admin could easily rename mydoom to a file and have it downloaded to your system.

While a solid firewall is a good step in protection you should really consider an anti-virus program. I use norton but there are a number of free ones available (google it or ask here). This way if it has autoprotection enabled you can tell when the virus is getting on your system and who may be respnsible.