how to know whether user's system is win 7 32 bits or 64 bits ..
I don't think there is a Qt-ish way of determining this information at runtime, but you might use native code instead, like "GetNativeSystemInfo()":http://goo.gl/lR0f4.
Do you want to know what the OS edition is (if so, do you just care about Windows?), or wether your program is running in 32/64 bit mode?
Second is easy using QSysInfo::WordSize, for the former you need OS-specific code (uname, Get[Native]SystemInfo, ...)