How to Reset User Profile of Built-in Administrator Account to Default in Windows

http://www.eightforums.com/tutorials/2755-command-prompt-boot-startup-windows-8-a.html
Command Prompt - Boot to at Startup in Windows 8

1. Do step 2 or 3 below for how you would like to open a command prompt at boot.

2. Boot from your Windows 8 installation DVD or USB thumb drive, or ISO file if in a virtual machine.
A) Press Shift + F10 when you are at the Install Windows screen, and go to step 4 below.

OR
3. Boot to the System Recovery Options screen.

4. The elevated command prompt at boot will now open.

http://www.eightforums.com/tutorials/38799-built-administrator-account-reset-default.html

   Note

If prompted to select an administrator account to sign in as, do not select the built-in Administrator account.

2. In the command prompt at boot, type diskpart, and press Enter. (see screenshot below step 4)
3. In the command prompt at boot, type list volume, and press Enter. (see screenshot below step 4)
4. Make note of the drive letter (ex: D) for your Windows drive, type exit, and press Enter. (see screenshot below)

   Note

The drive letter of your Windows drive will not always be the same at boot as it is while in Windows.

5. In the command prompt at boot, type the command below, and press Enter. (see screenshot below)

RD /S /Q D:\Users\Administrator

   Note

Substitute D in the command above with the drive letter of your Windows drive from step 4 above.
 If you had previously renamed the built-in "Administrator" account's name, then you will need to substitute Administrator in the command above with the new name instead. 
If your Windows uses a different language than English, then you would need to substitute Administrator in the command above with the translation for your language instead.

6. Restart the computer back into Windows, and sign in to any other administrator account other than the built-in "Administrator" account.
7. Press the Windows + R keys to open the Run dialog, type regedit, and click/tap on OK to open Registry Editor.
8. If prompted by UAC, then click/tap on Yes.
9. In Registry Editor, navigate to the location below. (see screenshot below)

10. Under ProfileList in the left pane, look for a long SID key like below with 500 at the end. (see screenshot above)

11. Right click on the 500 SID key, and click/tap on Delete. (see screenshot below step 9)
12. Click/tap on Yes to confirm. (see screenshot below)

13. Repeat steps 10-12 above for any other SID key with 500 or 500.bak at the end.
14. When finished, you can close Registry Editor if you like.
15. The built-in Administrator's profile has now been reset to default. The next time you sign in to the built-in Administrator account, it's profile will be recreated to a default state.