Is it running a machine? Do you have an archive file (*.acd)?
If you have a project file, you can open it in notepad and look at the first part of it which is a version history. The last entry in the list is the last time the file was saved. That will tell you the last version of RSLogix 5000 that was used.
If it's running a machine and you don't have an *.acd file, you can right-click on the PLC in RSLinx and pick "Device Properties". That will tell you the firmware revision. Once you have that version of software installed, you can upload the program. You will end up with tag names and code but no descriptions or comments.
If it's not currently running a machine and you need to program it...as has been said, you'll need v16 of RSLogix 5000. As Harrstein said, I would suggest a Windows 7 VM for v16 and WinXP for older versions. I've had v15 or so working ok in a Win7 environment, but v13 and older will not work with anything newer than WinXP. In my experience, you'll be able to install/run the software in Windows 7 but going online will not work. If the PLC is running a machine and isn't at v16, you'll need to install whatever version it's running to make sure you have a good copy of the program. I would then strongly suggest getting it to v16. We can help with that procedure if you haven't done it before. Best option, of course, is to move to an -L8xE series, but good luck getting your hands on one.
Is it using the serial port? If so...that's another complication with migrating it but that's the "next" problem and not today's.
