How can I fix a Windows File Protection error?

I have this new compaq mini notebook with XP SP3 running on it. I have the WFP error box popping at me nd all applications installed on it doesn’t run, for instance I installed eset nod on it but shows a “can not connect to kernel” error. All other programs like mozilla, microsoft office doesn’t run. I have done system restore but doesn’t solve d problem. I downloaded xp-SP3 to run but still the same and trying to downgrade to SP2, it complains of a much recent version alreadyy existing. Pls help!!