Your system seems to be infected by the Bugbear.B virus (its backdoor has been detected on port 81). More information: http://www.f-secure.com/v-descs/bugbear_b.shtml

Use your favorite antivirus to disinfect your system. Standalone disinfection tools also exist : ftp://ftp.f-secure.com/anti-virus/tools/f-bugbr.zip