Cannot install unity ever again

I encountered a problem so i want to reinstalled unity but when i tried it said that the media was damaged and that i should be secure i what i'm downloading and installing. i try with previous installion which worked before but now have the same result, my computer warn me to make sure the program i'm going to install is safe and all of this happen when i succsfully downloaded unity 4.5.2 and got this internal compiler error in unity which one solution was to reinstall to make sure you have the dlls. So i wondering if anyone can help me get a solution to this problem even thought there might not be