cannot access home network computer

Indeed all the same workgroup.

Let me give you more details:

Comp A:: newly installed XP professionel SP2
Comp B: XP Prof SP2
Comp C: XP Prof SP2
Comp D: Win 7 - Everyone has access. A shared printer is attached

Comp D can access files on A, B and C
Comp B & C can access files on the three other computers and use shared printer.
Computer A can access files on B and C but is denied access to D and cannot access the shared printer.
Clearly something is wrong with comp A, but what?