TampaPowers

I got OA gifted from a friend on gamersgate. I had Arma II on DVD for some time now. I downloaded OA and i manage to install Arma II plus the 1.07 Patch just fine. When trying to install OA i get an error telling me that a CRC did not match for file mics3.pbo.bi.bisign I looked up a couple of topics about this CRC errors, but none helped me so far. Most said about returning the DVD, but re-downloading the 7,5gb takes me about 18 Hours. I want to avoid that. What can i do?