Vista - Vista 64 bit not backward compatible with Fortran compiled at 32 bit

Dear all,
I have a problem in running a program made in Fortran 77 (by somebody else) and compiled at 32 bit in Windows on Windows Vista 64 bit.
I am forced to use the 64 bit because the 32 bit pc doesn't have enough ram for the operations required by the program.

I thought there was backward compatibility, but when i run the executable on Vista 64 bit, it tells me that it cannot do it because it is compiled at 32 bit.
I do not have a 64 bit compiler and don't know where to find it.

Do you know how can I solve this problem?
Is there a way to make Vista 64 bit running the exe anyway? (compatibility mode??)
Or should I find a compiler at 64 bit?

Thank you very much for your answer!