Xeon 64 issues

Question

I'm writing a C# application that runs fine on my 32-bit Intel Core Duo, but when I run it on a dual core Xeon 64, it crashes immediately, before even running any code. Is there a specific compiler option I need to enable, or something similar that I'm not doing? It doesn't even seem to be running any code. I have a debug line right after the main() function is called and it doesn't even get there. Any thoughts? Thanks so much