Your computer has been showing signs of a malware infection, and today it started up in safe mode. because your computer is not a member of an active directory domain, what all-powerful built-in account can you log on with?

You can log in to the administrator account.

Explanation:

Since your computer is not part of an active domain, you can log in to the admin account, its a built-in account in all Windows OS, this will grant you all privileges, and will allow you to perform any change you want to the computer. It will help you to find and effectively eliminate any malware present in the system.