iMac internal hard drive failed and needed to replace to use computer.
Had my 9 year old son help read instructions while i was doing the work, as my hands were busy. I sprayed out the computer from dust accumulated over the last 8 years, which took me longer than the 1 hour it could have.
When booting up, the hard drive wasn't recognized. I used the CD boot disk what was in the keyboard box. When I booted up, it didn't see the Seagate Hard Drive, so I used a USB flash drive. I installed the OS there and after a few updates and checking for the Seagate, it finally was visible to use. Then I used the CD again to put the OS on the newly installed internal Seagate Hard Drive!
We did have some complications with our Restore using Time-Machine. It had our user name /email, but the password was lost. Resetting the password worked and we got into the computer with our History still available for us to use.