Jeg fant ut av det. Denne framgangsmåten ved bruk av regedit fungerte:

http://geekybits.blogspot.com/ (...)

(Administratorkontoen kom fram igjen, alle filene var der, osv.)

Jeg kopierer over noe av teksten for å ha den på plass som referanse for ettertiden:

After installing Windows XP Professional and creating a user account, the Administrator account disappears. Also in Windows XP home the administrator account is disabled from default. However there is an easy way to get it back in both systems:

To add the administrator's account in Windows XP:


Open Registry Editor:

In the start menu click the run dialog and type 'regedit' (no commas)
In Registry Editor, navigate to the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList
Create a new DWORD calling it:
'Administrator' (no commas)
Assign the DWORD the value of 1:
Right click on newly made DWORD and select modify, then change the value to 1.

Now Close the Registry Editor and Reboot.

An administrator login should now be provided, and the Administrator account should show up in the Users section of the control panel.