Helping people with computers... one answer at a time.
If you have the password for the administrator account, you can easily elevate any other login account to have administrator privileges.
The administrator account is still there, it's just hidden, presumably for your protection. But we can still sign in using it, and then grant your other regular, account administrative rights.
But you will need the password for the 'Administrator' account. I'll assume you have that.
In Windows XP Pro, that Administrator account is not listed on the login screen, unless it's the only account on the machine. You can still log into it, though ... by rebooting into 'Safe Mode'. Press F8 when Windows begins to boot, and you'll be given a list of boot options, one of which is 'Safe Mode'. Select that, and the 'Administrator' account will appear on the login screen.
From here it's easy. Once you've signed in as 'Administrator':
Right-click on My Computer
Click on Manage
Expand Local Users and Groups
Click on Groups
Double click on Administrators
Click on Add
Enter the account name on this machine that you want to assign administrative privileges to.
OK your way back out.
Now reboot your machine normally, and the account you selected should now have administrative access.