To start off it would be nice to know whether you have a Retail or OEM Windows 7 disk. The rules and methods are different for each.
Microsoft checks whether it's operating system is being installed on the same or different computer by the motherboard.
The basic Microsoft rules.
Retail Microsoft operating system (COA) is allowed on one computer at on time. You can install the Retail on as many computers as you care as long as it is one computer at a time.
OEM Microsoft operating system (COA) on one computer. The OEM (COA) is tied to the motherboard. When you change the motherboard Microsoft considers it another computer. Their for another (COA) would have to be purchased. If you don't change computers or motherboards often OEM works just fine. If you do a lot of replacing I would recommend a Retail version.
Either way I would recommend backing up everything you want to save and copy down all authorization codes for other programs you may have. Then I would do a clean install with the proper COA as per our Tutorials. Clean Install Windows 7