1) A single Windows 7 key gives you the license to install only on a single PC. So you are violating the EULA the moment you try to use one key on multiple machines. Also, retail licenses (the kind you buy from a store in a box with a dvd key) are freely transferrable as long as you use it only on ONE computer at a given time. But if you switch hardware frequently, the online activation servers will not like it and fail you. In that case you need to call MS and do a phone activation. Activate Windows 7 by Phone
2) If you need to run Windows 7 on multiple computers, you can buy a 3 user pack for $150 if you are a end user or contact MS for an enterprise license if you are a network admin.
3) What is this about cracks? Thats what your parents taught you?