What was the reason you wanted to reinstall from the 17", were you having problems with the 20"?

When you say reinstall do you mean the "restore from a backup" option or were you using migration assistant to bring the files from the 17" over to the 20"?

What version of the OS was on the 17" iMac and was it a G5 or Intel?

Do you have a spare external drive that we can wipe and use?