Good enough, that’s not the problem anyway then.
Perhaps the file is in fact corrupted. Please browse to http://www.axantum.com/AxCrypt/Downloads.aspx and then chose the ‘AxDecrypt.exe’ download you’ll find there. That’s actually the exact same software that’s part of the self-decrypting exe, just without any payload. It, however, has the capability to decrypt another file even when it’s an ‘exe’.
It has a very sparse user interface, but if it runs ok, use the File | Open menu, then select “*.exe” as the file type to open and then find and select the file you’re trying to decrypt, and then click ‘Decrypt’.