xp machine can’t get back on the domain

I have A computer on a network of over 50 users,running win ser 03.one day a user reports he cant get on the network.when i check the computer had some trojan of which i remove.network card/loopback address is fine,cable is fine and just cannot get the computer back on the network.we do use ics so the computer is able to get on the internet.
reformating the drive is my last resort.maybe some of you have come accross this problem.