After assembling the new system,
connect your old drive. Do NOT connect any other hard drives.
Boot up the system. You may need a couple of boots but all will be ok. After system booting successfully, use SeaVixen's advice IF you are prompted to activate or get some other dire Microsoft warning.
don't worry about installing drivers
Just go to update and keep running update until all drivers are up-to-date.
If you have any problems, let us know. Win 7 is more flexible in this regard than earlier versions.