Which version of Windows will run on an iMac 24" early 2009 model?
I managed to install Windows 8.1 Pro on my early 2009 24 inch iMac yesterday. Just a disclaimer: I DID NOT USE BOOT CAMP ASSISTANT TO INSTALL. For 2 days straight I troubleshooted boot camp assistant to try to get it to take my Windows 8.1 Pro to no avail. Out of frustration I googled 'how to run windows on a mac WITHOUT boot camp.' After finding a guide online, in a matter of about 30 minutes I had successfully installed Windows 8.1 Pro on my 2009 iMac. Here is the link to the guide....you can ask me any questions regarding this. 
See "How To Install Windows 8.1 On Mac Without Boot Camp".