Originally Posted by IRLegend
This sounds like a serious bug. You should contact microsoft about this. I would also try booting in safemode and seeing if that helps you to access an admin account.
It is also possible you have modified the local security policy to require admin credentials on elevation requests even for admins. When it asks for an admin username/password have you tried just clicking OK?
Also - Usally when there are no admin accounts active, the system will activate the built-in admin. Can you see that account on the logon UI? If you can use that account to reinstate your old accounts priviledges. The default password is usally something like "password" "administrator" or "admin" depending on who supplied your computer. If someone installed the OS for you, only they will know the password.
There is also methods in which you can reset the admin accounts password, however I dont think I am permitted to explain that on these forums as it could be seen as hacking
(You can't modify the security policy in home versions)