This is a full in depth tutorial and is part 1 of 3 tutorials.
Method 1 in this tutorial and
Method 2 in the next tutorial.
Part 3 will show you how to get around locked desktop backgrounds and more. This is for Windows 7 computers however I have other tutorials that will work on Windows 8 and Windows 10 linked below.
Method 1 Updated for Windows 7: http://newhackingmethod.blogspot.com/2016/05/how-to-access-admin-accounts-at-work_3.html
Method 2 for Windows 7/8/8.1/10: http://newhackingmethod.blogspot.com/2016/05/method-2-how-to-access-admin-accounts.html
Method 3 for Windows7/8/8.1/10: http://newhackingmethod.blogspot.com/2016/05/how-to-access-admin-accounts-at-work.html