assuming the OS is win7 and the machine is not a works or school machine
gaining full admin privileges can be gained following these steps
1.Click Computer
2.Right click on the Hard Disk icon where your OS is installed on and click Properties.
3.Click the Security tab.
4.Click the Advanced tab.
5.Click the Change Permissions button located after the Permission Entries list.
6.A new window will appear on your screen. Such a window contains a list of all the user accounts available on your computer.
7.Select the user account you want to give total control over your Windows 7 and click the Edit button.
8.Now, tick the checkbox labelled “Total Control” and press OK.
it may take a couple of changes to for the configuration to set.