Have you applied any updates? I applied that latest patch (1.1.0.8 i think, but not 100% sure) before I played the game on XP and it worked ok, so i dont know if this will cure your particular problem.
However, Opposing Force wouldnt work before I applied the latest OpFor patch (which happens to be 1.1.0.8 as well, but they are two seperate patches i think)