In Windows 7, how do you change an existing "standard" user account into an administrator account without removing and re-adding the account?
There are a couple ways in which you can do this. For both methods, you will want to log in under an existing user account with administrator access.
1) The first (and simplest) way is through the User Accounts control panel.
First, open the Windows 7 control panel, and click on User Accounts and Family Safety. Under this subsection, you will see a section named User Accounts. Under this section, click on the link titled Add or remove user accounts. (If you have UAC enabled, it may ask if you really want to proceed, and may ask for a password. Follow the prompts to continue.)
In the new screen that appears, you will see a list of all user accounts on your system, as well as an option listing the Guest account. Select the user account that you wish to change. (Note: You will see it listed with the label "Standard user" under the name of the account.) Once you click on the user, a screen with various options will appear. Click the link that says Change the account type. You will see a new screen with two options: Standard user (which should be selected), and Administrator, with some description text under each. Select Administrator and click the button labeled Change Account Type. Now, the account should be an administrator.
2) The second method involves the use of the Computer Management console.
Open the Start menu and search for Computer Management in the search box. An item under the "Programs" section in the search results called "Computer Management" should appear. Click on it to open the Computer Management console. (Again, if you have UAC enabled, it will prompt you for permission. Follow the prompts.)
In the console window on the left, there will be a tree of items. Under the System Tools section, click to expand the Local Users and Groups item, and then click on the Users sub-item that appears. Find the user in the list on the right that you wish to grant Administrator access and double-click it. A window will pop up with the user's various properties listed.
You should see a Member Of tab on the top of the window. Select it. This will display a list of all groups the user is a member of. If this user is not already an administrator, the group Administrators will not be in the list, so you will want to add it.
Click the Add button and a "Select Groups" window will pop up. In the text field (labelled "Enter the object names to select"), type Administrators and click the Check Names button. The text you typed should change to reflect the Administrators group on your machine:
Click the OK button in this window, and then click the OK button in the User Properties window. The user should now be added to the Administrators group. Once that user logs in again, (s)he should be able to perform administrative tasks with his own user credentials.
WARNING: You MUST have at least one user account on your computer with Administrator access, even if you do not intend on using it regularly. This should be separate from the hidden Administrator account or any other system account that you did not create yourself. If you remove administrator-level access to all of your user accounts, you will lose the ability to change any system settings on your computer. Recovering administrator access to your user accounts when this situation has occurred is more difficult and potentially dangerous for someone that is inexperienced with computers. Beware.