Used and Abused

I have followed Microsoft's instructions for creating an answer file on a 'technician' PC and am now trying to create a Client PC using a Windows 7 Professional Upgrade DVD and my answer file on the USB stick... all seems to go great apart from the fact that because the school I work at bought an WINDOWS 7 UPGRADE DVD rather then a straight from scratch installation DVD I have to log into the current XP OS before it will run the reinstall, the site advises that it must be run from boot in order to use the answer file that I have just spent 4 hours perfecting... Soooo... i suppose my question is this... how can I install Win7 Prof as a clean install using the answer file i just created? after the client PC is perfected I am looking at using WDS from the server using pixboot...