I guess you can try Windows 8. Windows 8 (supposedly) allows for 5 GPUs without any special settings or drivers. Wouldn't hurt to look into it.
I would highly advise not using Windows 8 for mining the cat drivers for Windows 8 are broken and skd where it installs goes to wrong locations and also cat install installs the OpenCl driver needed however places it in wrong folders for install graphics drivers.
You can do it on Windows 7 depending on how you have set this up and have it configured. You might need to tweak around in the bios slow some clocks and timing to get all 5 cards working from the PCI-e slots then ull need some extenders to allow extra room and allow good air flow...
Regarding you putting them into cross fire you don't need to do this. This is more than likely the problem that is the cause. Remove the crossfire cables and have 1 monitor connected to the first card and boot the system up with the others in. Use cgminer set up your configuration for them and it should auto detect all the GPU'sIf this does not happen you might need some dummy plugs however shouldn't need them nowadays.
If it does not work then you maybe have to change bios settings or finding a custom bios Am not full up on the Asrock boards but I would have a play around b4 switching to linux but then again running on lunix might be better too.
