There is no supported default Administrator password to use here.
For local accounts, the built-in Administrator account cannot use a blank password as a security rule. On many Windows installations, the built-in Administrator account is also disabled by default. That means the password prompt seen from another PC is not something that can be solved by guessing a default admin password.
For access from another computer on the local network, the simplest supported fix is to either:
- Use password-protected sharing with a valid user name and password on the target PC, or
- Turn off password-protected sharing on the target PC.
To turn off password-protected sharing on the new computer:
- Open Control Panel.
- Open Network and Sharing Center.
- Select Change advanced sharing settings.
- Expand All Networks.
- Under Password protected sharing, select Turn off password protected sharing.
- Save changes.
If password-protected sharing is left on, people on the network must use a user name and password that exists on the computer hosting the shared folders.
If the goal is specifically to set a password on the built-in Guest account, the documented method is:
- Sign in with an administrator account.
- Open Command Prompt.
- Run:
net user guest * - Enter the password, then enter it again to confirm.
If the goal is to use the built-in Administrator account, note these security points:
- The built-in Administrator account cannot use a blank password.
- Best practice is to sign in with a normal local account and use administrative credentials only when needed.