It's currently a 32 bit program. Though there will likely be a 64 bit "beta" version sometime within the next year or so.
Functionally, there won't be any real difference in program behavior. Windows will treat us differently, but just about everything should behave the same.
The main difference would be for large models where Windows won't allow you to solver for more than "X" number of load combinations because of 32 bit memory restrictions. That should be the main noticeable difference.
There should not be any problems with the current versions of RISA.... At least not because of the 64 bit operating system. We've done a lot of testing of these versions on 64 bit machines in our office. Windows 7 is a bit more "persnickity" than XP was, so there are sometimes rights / permissions issues. That's why we don't recommend installing the program into C:\Program Files\.
If you're running an older version, then issues could arise because we didn't do any testing for 64 bit until a couple of years ago. Though any issues with the older versions are more likely to be related to the driver for the hardware key or with the install routines.
