How do i login on as administrator on my computer?

Do you have the administrator access?
If you don't, you might recover administrator login password.

Login With the Administrator Account in Windows XP Professional

Click the "Start" menu and then click "Log Off."

Click "Switch User" when the prompt screen opens.

Press "Ctrl," "Alt" and "Delete."

Click on "Administrator" when the "Log on to Windows" screen pops up. Type in the administrator password, if applicable.

Click "OK." You will now be logged in as the administrator.

Log On as An Administrator in Windows Vista

Start your computer and allow it to continue through its startup process. By default your computer will simply load directly to the primary user account (usually the administrator account), but if there are multiple users on the system it will load to the account login screen.

Use the mouse to click on the administrator account from the list of available accounts to log on. This step is only necessary if you've customized Windows Vista with more than one account or you have set the operating system to load the login screen first.

Enter the password required in the password field to log on to the administrator account.

Click the "OK" or "Log On" button once you have entered the password. This button may be labeled differently depending on the version of Windows Vista or any GUI (graphical user interface) changes you may have made.
Log on as an administrator from within the Windows Vista environment by clicking on "Start" and then "Log Off." As soon as you have logged off, you will be returned to the login screen where you can choose the administrator account from the available list.

If this answer helpful please vote me.

Good luck.

Thanks for using FixYa

Log in to your current account and hopefully it has "administrator" setting - it must have this in order for you to create the Administrator account. Go to control panel and select User Accounts. Select Add or Remove User. Select Create user account. Create account with account named "Administrator" then click the administrator circle check.