If you go to User Accounts in Control Panel it should tell you (under each user's name) if they are a Computer administrator. Any user that is a Computer administrator can run Defrag etc. The Administrator account is not listed here.
If no user has administrator rights then you need to login as Administrator and then you can assign administrator rights to any user that you like. The procedure for this is different for XP Home and XP Pro.
For XP Home you can only login as Administrator by restarting the PC in safe mode. It is normal for the password to have been left blank so just press Enter when prompted for it.
For XP Pro you should log off to get to the login screen. Once at the screen press CTRL+ALT+DEL twice and you will get a classic login box. Type Administrator as the Username and type in the password that was set during installation or leave that box blank if none was set.