Becouse if u have Windows 64-bit, then some program( 34-bit programs) doesnt wanna start..
And MTA 0.5 isn't included in that list. I can confirm as a 64-bit user of Vista Ultimate.
[and theoretically Windows itself supports all 32-bit applications, but 16-bit support is completely removed from 64-bit versions]
If Windows says that it's not a valid Win32 application,
the file gets corrupted somewhere along the way. Try disabling your antivirus while installing.