This has been discussed here before. Search is your friend:
What version Windows do you have? I am pretty sure you set up your computer initially with a user and password and then turned it off...
Click the Windows windows key Start menu.
• Type Control Panel in the search box, then select Control Panel in the search results.
• In Control Panel, open User Accounts.
• Click on Manage User Accounts.
• All user accounts will be listed on the page. Each account is part of a group (Administrator, Standard, etc).
Or try this way:
Determining your Windows Username | Technical Support Services
See these threads as well...
Local Console Login?
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