Need some help with a trojan.

[RiversAreBad]

Recently I got a trojan on my desktop and ran both MacAffe and Norton to get rid of the problem. They both said they found and cleard some trojans, however now when I turn on my computer it is very slow booting up, it says my task manager has been disabled by my administrator and I cannont get the screen resolution above 800x600 with 4bit color quality. Anyone got any ideas?