"What you have is a licence key to use Windows 7 pro - not to use Windows 7 pro x86.
The serial keys are unique to the edition (Pro, Ultimate etc.) of Windows but not the platform (x86, x64).
Out of the box by using the disk alone, you can only install the edition that the disk is designed for, but it contains every edition, but only a single platform (Off topic - if you want to install a different edition, look at the Windows Automated Installation Kit / WAIK, as generating your own auto install file will overwrite the one on the disk.)
If you go to wherever you got the Windows 7 disk from, you should be able to get the alternate media that will cover you for x64."
