Windows 7 on multiple mach's?

Can one copy of one Windows 7 be installed on multiple mach's or a way to trick it to allow? Or has Microsoft made it check the serial # of the hard drive and verify it has not been installed all ready or some thing like that? I ask when Window 7 shopping at my local Best Buy I saw several different versions one being Family pack that says on the box may be used to installed on up to three mach's. I want to get Windows 7 for my own four home use computers but no way for all of them if I have to pay for it more then once. I see it is I purchase the software for my own personal use I should be able to use it for my own personal use on at least a reasonable (more then three) mach's. From the little I saw on the boxes of Windows 7 I would get the Ultimate Windows 7 version cause it allows programs to be used the way XP does in case the program being used only works on XP but it does not say one the box it can be used/installed more then once. Ya ya let the rants begin. :mrgreen:

 

Microsoft sell a 150us$ version that can be installed on 3 computers. Just buy more copies in case you need to install it on even more computers.