I have always recommended that people handle the "administrator" password with the utmost sanctity. This account should, in a best case scenario, be used one time only -- the first time you log in after a fresh install. At that point a second administrator level account should be created, and you should log back in as that new account to do everything else. The administrator password should then be written down and locked in a safe so that if it is ever needed, it can be readily retrieved. Now with that said, here are the options for recovering it.
First you can try guessing at it, especially if it's a password you created. You might get lucky, especially if you have a method to how you pick your passwords. If that doesn't work, you have to use a tool-based password hacking method to recover the password. There are a number of utilities available in both fee and freeware format. Probably the best known is @stake LC5, which is the newest version of L0phtcrack. In the realm of free tools you have a number of options:
1. John the Ripper
2. Offline NT Password & Registry Editor
3. Emergency Boot CD
4. Knoppix Boot CD
Any of the above should do the trick, and in the case of Knoppix for example, it contains a number of other handy security and hacking-related tools and utilities.
BE ADVISED -- One word of caution: Resetting the administrator password can cause data loss, in particular if you are using EFS.