neeed help

hi everyone, i have a serious problem with an computer.
winXP, SP2(hacked), virused… quite how to can’t instal avast on it. i tried mbam, killer rogue. which founded a lot of virii, including trojans, but i still can’t instal avast.
also, an virus which make faked files is still on it, i saw this with an memory stick.
i can’t attach the results because i’m online with the mobile and i don’t have internet on that computer.
any ideas to how instal avast pleassse?

Please follow the instructions here for infected systems http://forum.avast.com/index.php?topic=53253.0 then start a new topic in the viruses and worms section including the logs requested in the link posted.

Thankyou