unity3d standalone runs on one machine but not another?

My friend has a standalone that he built that was working fine. It will no longer run on his machine. He cannot build it either. HE sent me the project and I can build it and run it. When I send him the build back, it won't startup on his machien. What could cause this problem?

I don't get it. It can't be the player because the standalones don't use a player right? What's corrupted? HOw do I check it. The player log shows nothing. He's using unity 3.0 I think.