Did you buy the windows 7 "retail" pack with the box?
This comes with 2 CDs, one says 32bit and the other says 64bit. You can only install one of them at a time and they use the same code that came with the box.
It sounds like you have the 32bit version installed so you will have to do a wipe and re-install windows with the 64bit CD. (Remember to back up anything you want to save)