According to their web site, FastStone Image Viewer is compatible with Windows 7 and with 32 bit and 64 bit operating systems.
However, if you are using an older version of FastStone then that may be the problem;
If your version of FastStone is earlier than 4.2, try upgrading. I didn't see any upgrade instructions in my brief search there and if there are not any, then my advice is to uninstall your current version, reboot the computer, (to update the registry and other system files), then install the new version.
If that fails, then try another product. We often recommend Irfanview.
I've tried searching for an answer to this as a general problem, not just with the program I'm having trouble with. If it's in this forum, I can't find it.
I'm running Win 7 Home Premium 64 bit on my Dell PC. This is a new computer and the OS was pre-installed. I can't see any of my photos using FastStone Image Viewer, which I've been using for years. I get the error message "...not a valid Win32 application" Is there a way to fix it so that it's compatible? If not, what photo viewer would work with the 64-bit system? I hate using MS Picture Viewer.