Bro download unarc.dll file and past this file on go c drive. GO windows folder. And find syswow64 folder and past in this folder. Same problem was with me. But after that I installed completely.
Restart the PC and wait for 30 minutes then try it. Or you could log in via safe mode then do a normal boot. One of these worked when I was testing witcher 2 before I bought it. But if it works by it!
Thats what usually happens when u try to run a pirated game.your setup file probably has some kind of corrupted file for which reason it can't install. Only solution is re-download which is a big pain in the ass.