computer finished?

hi folks this is my first post as ive never had a problem i couldn't fix before. This time however i fear its too bad. It started off when i wanted to change my anti virus from agv and zonealarm to kaspersky. When doing this at some point i downloaded a virus. It wouldn't let me run any anti virus software at first but i managed to get through that. Did all the READ ME FIRST before posting. But now i cant even access explorer. Cant boot with xp disc and cant access the net. (using mobile to write this,) i think the virus attached to userinit.exe and user32.dll. Tried xp disc to expand to windows system32 but couldn't. Now when i boot up i get my background pic but no explorer and no task manager. Also can't boot from xp disc or in safe mode. HELP!