Wndows 7 activation?

To avoid the OEM upgrading issues I bought a full copy of Win 7 to install on one of my desktop PCs when I reformatted the OS drive to start from scratch.

I now want to replace that computer with a different box and wondering if there's a way to "transfer" the OS. If I just try to install Win 7 from the full version disc on another computer, I assume it will see that's it's already been activated?

Any way to legally return to XP on the old machine and install Win 7 on the newer one?