Hold Option when it boots until you see a list of boot partitions. What do you see?
OK choose Recovery HD then open Disk Utility. Select the partition (usually Macintosh HD) right below the device name and then select the Erase tab and Erase. Then quit Disk Utility and reinstall Mac OS X
You should be able to give it your new username and pasword
Choose OS X and see if you can erase that, rename it Macintosh HD if possible
If that won't erase try 1. Shut down the computer.2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need them in step 4.3. Turn on the computer.4. Press and hold the Command-Option-P-R keys before the gray screen appears.5. Hold the keys down until the computer restarts.6. Release the keys.
Then see if Macintosh HD shows up now
Yes now quit Disk Utility and try installing to Macintosh HD
You may have to Choose Network
Yes, let me check
It may be locked to the Apple ID if you can't change it. Are there any other options?
How do I do this?