Hello Syrus,
1)Does it happen in Single player too?
2) Tried running it as administrator? "RMB(right mouse button) and click "Run as administrator"
3) Illegal or legal copy. Truth please, because then I can really evaluate the problem.
4) Windows 7 is your OS, correct?
5) Try installing the latest .NET framework version (Again, if you have it already)
6) Last but not least re-install IV:MP, delete all its folders and registry keys. ( Install again, afterwards)