A lot of Internet bandwidth is being used even when I'm idle?

I've a laptop with XP professional version 2002 Service Pack 3. I've been using reliance netconnect (USB modem for internet). I've Zonealarm free firewall, Spyware Gaurd and Malwarebytes Antimalware installed in my system. I've been infected by some trojan and has scanned my system with Kaspersky online scanner. I've performed a full scan with Malwarebytes and cleaned the trojan. But now it is taking quite a long time (freezes sometime at 'loading your personal settings' and only wallpaper appearing without any icons or taskbar for sometime) for startup and also I observe a huge use of internet these days. I just sit idle and internet is being used. I've a bandwidth limit of 5GB per month and in a 15-18 days all is becoming exhausted. I'm not viewing any data streaming websites like YouTube or large sized websites. I guess this is due to a lot of service applications running or it may be some virus stealing all my bandwidth. My upload is very high (sometimes equaling my download) and this seems very suspicious. In a half-an-hour the bandwidth has reached 95MB without watching any video sites and without downloading anything. Sometimes a blue screen appears stating that "Your computer has encountered a problem and to prevent damage it is restarting it". Any help is appreciated..Thanks.