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When you are ready to install, execute this psexec command: (Replacing yourdomain, adminuser, yourpassword, and yourserver with your values)
c:\psexec -u yourdomain\adminuser -p yourpassword @hosts.xml -d msiexec /i "\\yourserver\wpkg\WPKG Client 1.2.1.msi" /qb SETTINGSFILE=\\yourserver\wpkg\settings.xml
This will then install WPKG on any of these clients that happen to be on at the time :)