it's even possible to have both systems installed but makes sense only if you use applications which don't run under win64 such as itunes or eos utility..
I don't know about iTunes, but EOS Utility (as well as DPP and ZoomBrowser) will install and run in Win7 64 although it may not utilize the full 64 bit capabilities.


However, When I wanted to stack them in PS, it gives an error " Could not bla bla..., out of memory" How can I fix it? Or any other stacking program ? (I have tried ZereneStacker and CombineZM but these are not successful like PS)

