Boot from the Tiger disks that came with the Mac.
Do a format and install.
When done, create the admin account, reboot and upgrade to Snow Leopard
P
The iMac is my wifes'. I know what to do with Windows, but can't fix this. The iMac was new in 2007, came out with Tiger OS. In 2009 we upgraded to Snow Leopard. It ran fine. Couple of months ago, We upgraded to Mountain Lion. She has never liked it. We still have the DVD for Snow Leopard. If it was a p.c. I would just DBan and re- install the OS of her choice. The QUESTION is ... can I use this DVD and re- install Snow leopard? Thanx in advance

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