RIS – with Removable Hard Drives

I would like to deploy RIS at my school. Students use removable HD’s and two students share the same computer. When the first student installs W2k on their system everthing goes well. However when the second student tries to install on their HD using the same computer, the install fails. I’m assuming this has something to do with the machines GUID. How can I associate 2 user accounts with the same GUID under RIS?