For Windows versions all the way up to XP, you were supplied a built-in administrator account that was always there by default. For Windows Vista and later systems, this was no longer the case. This post describes how to activate this administrator account from the command prompt in Windows Vista systems and later.
Update: to further clarify, this admin account and the guest account is still created in modern Windows systems, but they are inactive by default.
1. Right click on cmd.exe to run command prompt as Administrator
1a. Make sure to run the command prompt as admin or else you'll get the error – System error 5 has occurred. Access is denied.
2. Type the following:
net user administrator /active:yes
3. To deactivate, simply type:
net user administrator /active:no
Note: you can also run the command prompt as Administrator by typing
net user administrator yourpassword.
Warning: only use this built in admin account for troubleshooting purposes. If you enable it, disable it when you are done. The permissions are a little more elevated and a password isn't required to login. In general, if you have to ask if you need it, then you don't need it. If you do want to keep it enabled, at least create a password for this account.
This one is pretty simple. There's not a whole lot of deviation in activating the built-in administrator account from the command prompt. Again, don't leave this on when you're done, as this would leave a huge security gap in your system.
On an interesting side note, I actually don't have net user listed in my Useful Run Commands for Windows post. At least I do have 80+ other entries though, including lusrmgr.msc – Local Users and Groups. This interface shows the built in Local Users and Groups tool for those who want a visual way to manage accounts instead of using the command line.