You installed the Windows 7 you bought on your Vista machine.
If you now wanted Windows 7 on the XP machine, you would have to uninstall the Windows 7 from the Vista machine to be able to install it on the XP machine and you can only transfer it from the Vista machine to the XP machine if the Windows 7 was a Retail License.
And One Windows License is for ONE installation on one machine.
Microsoft no longer sells Windows 7, you need to try at Amazon.com if you need to buy 7.
Follow these steps before you buy Windows 7 or are able to transfer it from the Vista machine:
Go to your Computer / Laptop Manufacturer’s website and see if Windows 7 Drivers are available for your Make and Model Computer / Laptop.
If not available, Windows 7 will not work correctly for you.
Run the “Windows 7 Upgrade Advisor”
Check to see if your System Specifications are compatible for Windows 7:
“Windows 7 system requirements”
“Windows 7 Compatibility Centre” for Software and Hardware:
Windows 7 Upgrade paths:
“Installing and reinstalling Windows 7”