I gather from other postings on Apple.com that I probably should have completely wiped the hard drive of my used IMac before installing my own files. I may still have to do that. I tried shutting down Time Machine, which I am using with an external drive. The previous owner apparently did not have an external drive.
Erasing is rarely necessary.
But make sure your backup is not the only one that you have before installing anything is necessary.
Note, if you have a G5 or G4 Mac you can't upgrade to Snow Leopard.
If your intent is to upgrade to 10.7 or later, see this tip: