You bought a retail copy of Windows which belongs to you for life, to run on your choice of machines as long as it is not on more than one machine at a time. Only factory or builder's OEM copies are locked permanently to hardware.
When you reinstall 64 bit (you can have either at any time), just wait until installed and insert key at Computer>Properties link.
It may auto-activate since it is on the same hardware, however there is a possibility that you will need to make a quick robo-call during which you will exchange a series of numbers that deactivates the old installation and activates the new one - this is normally when installed on a different machine, though.
If you ever have to speak with a MS person during the call (I haven't) then just explain what you are doing - possession of the card key insert and receipt will rule in the end.
Amazing how people think MS can deny activation on a whim. You bought it and it belongs to you for life. They are only concerned that it is on one machine at a time.