Note: Some password recovery tools is the list I linked to above also have the ability to transform regular user passwords into administrator passwords. This could be valuable if you know your account's password but it's not an administrator account.

Perform a clean installation of Windows XP. This is a last resort option. This type of installation will completely remove Windows XP from your PC and install it again from scratch.

If you're just curious about your administrator password, I wouldn't suggest this extreme solution. However, if you're needing the administrator password to access diagnostic tools and this is your last effort to save your PC, performing a clean install will work. Keep in mind that it is a time consuming process due to the fact that all of your data must be backed up and then later restored.