When I installed Calibre 4.12 my win7 wasn't too old. I stopped updating Calibre when it wanted a later version. If you want to do a "new" install on Win7 then the recommended version is 3.48 I believe. FYI previous versions of Calibre can be found
here. I've not bothered to go up from 4.12 as it is working ok and I don't want to break it now 4.x is unsupported.