My ten cents: Unless you replaced the motherboard with another of the same model (in which case you wouldn't be asking the question) this is not a good idea. Windows installation puts in things that are specific to the chipset and other little doohickies on the motherboard. You're in a sort of Catch-22 trying to work around it after the fact.
Even before the fact it takes a great deal of experience to be able to guarantee that the same install of Win(X) will boot on a new motherboard. Note that I wrote "guarantee," many people try and a good number succeed.
If your goal is really to ensure that you won't lose any data: Buy a new drive, install Win7 on it fresh, then attach the old drive to your system and either copy the data from it or use it as a data drive.
I may now be flamed by more experienced members.