I heard that software companies make these requirements in order to prevent people from calling tech support and saying things like "Windows is too slow!" or "Why don't DVD's play well on my spankin' new 486?"
Another interesting case is Mac OS 8. The setup requires that you have a 68040; however, the utility "WishIWere" lets you bypass the requirement and install it on a 68030.
To bypass the system requirements that Windows (not sure about Windows NT or 2000) imposes, just start the setup program using the /nm command-line argument.
Much of this info taken from www.emulators.com