I’ve been using Cubase 7 on my Windows XP at work since Tuesday, and works WONDERFULLY. I have no problems with it, at all.
I don’t know why the installer doesn’t support Windows XP but the software clearly does (and I hope it will for a long time). I’m fine with Steinberg dropping support for an obsolete OS, but at least give us the chance to install the software at our own risk! I certainly won’t complain 
At home I use a Windows 7 64 bit OS.