An administrator account is required to install programs and make changes to most of Windows' settings. If you have your own computer, chances are you already have an administrator account. If not, you will need to log in as an administrator to perform the administrative tasks you want.
Steps
Method 1 of 2: Windows XP Home

Step 1. Boot your computer in Safe Mode
If you have Windows XP Home Edition, it is only possible to use the built-in administrator account via the Safe Mode login screen. To boot into Safe Mode, restart your computer and hold down F8. Select Safe Mode from the boot menu that appears.
If you are the only user of the computer, chances are you already have an administrator account. You can check this by going to the Control Panel and clicking User Accounts. Find your account and the words "Computer Administrator/Administrator" in the account description

Step 2. Select the administrator account
When the Welcome screen appears, you will see a user labeled Administrator. Click on it to log in as an administrator.
- Most users don't have an administrator password, so leave the password blank for a while.
- If you did enter an administrator password when installing Windows, you will have to enter it to log in.

Step 3. Reset your password
If you have lost the administrator password, you can run a recovery program to gain access and change your password. See wikiHow for more info on downloading and running OPHCrack, a password cracking tool.
Method 2 of 2: Windows XP Professional

Step 1. Open the Windows welcome screen
Click Start and select Log Out or Switch User. This will take you to the Welcome screen, where you can select a user.
If you are the only user of the computer, chances are you already have an administrator account. You can check this by going to the Control Panel and clicking User Accounts. Find your account and the words "Computer administrator" in the description of the account

Step 2. Open the Windows NT login window
While in the Welcome screen, press Ctrl+Alt+Del twice to open the Windows NT login window.

Step 3. Enter the administrator account information
If you have created an administrator account, enter the username and password to log in. If no administrator account has been created, type "Administrator" in the "Username" field and leave the Password field empty.