Paul Thurrott recently wrote up an excellent article about how to replace your hard drive, but keeping your OS install completely intact. The secret is using Windows 7′s backup "system image" feature to create a complete, bootable image of your old drive, then restore it onto the new drive. Check out the full article for the details.
But I always end up making things more difficult for myself. When I was faced with a similar situation, I ended up taking a different route.