When it boots, if it has multiple user accounts, it should list them and allow you to select one. That will be labeled with the name of the account. On a machine with a single account, does it log in automatically? If so, you can go to control panel and then user management to see the accounts on the machine. Alternatively, you could try booting it up/logging in as the local admin and finally, you could take the hard drive out of the machine, connect it to another by SATA or an external drive caddy, and then look inside the C:\users directory to see the user folders on the system. The names on the user folders are the same as the account names.