I have this dowloader who sent tons of fax using my internet connection. He plug himself and my computer is now a real calling network. Neither my internet service provider, neither microsoft wants to help me out. For ...sake, i am a student and don't want to continue paying a huge phone bill.

My first suggestion would be to try firewalls, one hardware (like SMC's Barricade), and one software (here's a free one: http://www.zonelabs.com/store/conten...eDownload.jsp).