cannot connect XP client to SBS2003, cant detect network location

After sorting out a nasty trojan dls0523, repairing the winsock stack to get internet back, I am severly stuck….

I can ping the server from the pc, but cant ping pc from the server. The GPO will not update, the password accounts on the PC no longer work and although all the manual settings are correct for connecting to the network I get the message network location cannot be reached.

Any ideas – please!!!