Team Fortress 2 - Steam Validation Rejected?

I bought the Orange Box pack when it was for sale and I played Team Fortress 2 a lot. So I downloaded it on another computer so I could play with a friend. But he didn't have a Steam account, so he downloaded an illegal Steam. So now whenever I try to use this computer to play TF2, it says Steam Validation Rejected. How can I switch back to the real and legal Steam?! Please help. Thanks.