UPDATE:

I don't know if this is relevant. I right-clicked the installation file and extracted the files. They are:

AppleMobileDeviceSupport.msi
AppleSoftwareUpdate.msi
Bonjour.msi
iTunes.msi
MobileMe.msi
QuickTime.msi
SetupAdmin.exe

I suspect that the final one is that which starts the whole installation process. I'm not familar with .msi files, other than knowing that they're used in a domain to deploy applications via Group Policy. Is there some way of easily examining the files to determine why the process seeks J:?

I'm happy to go with the virtual drive technique that I mentioned in my first post if that will be easier and won't mess up my pal's system.