You can't upgrade XP to 7 directly you need to install a fresh copy.
You can setup a Windows 7 base image and restore it on several PCs, this will require some configuring on your part though.
This link has a pretty simple guide, and there are some links at the bottom of that page that can offer you more guidance. http://www.intowindows.com/how-to-create-unattended-windows-7-installation-setup/