[cc1834]

ok well i just ought a used g4 i mac, it was previosuly owned by a best buy i believe, the user name is best buy, but i however do not know the password, i need to know how i can change the user name or password, also, it says there is a disk in the disk drive callled "demo disk" and when i try to eject it it says it is in use, and will not let me eject it, any help whould be greatly appreciated

[Nivag]

to eject the cd, either hold down the mouse button or the eject key on the keyboard on starting the Mac.

to change the username & password you'll need a copy of the Operating System on cd.

once you've got the other cd out, insert the OS cd, restart with the 'c' key held down.
The easiest way for you to change the username would be to do a clean install, that way any problems that might be on from a previous owner wont occur and troubleshooting from a clean install is easier.