If you just can't remember the administrator password, search google for "linux boot disk NT password" and you'll find plenty of sites with descriptions of how to reset the password.
WinXP also has an option to "repair" an installation (select it when Setup locates the existing installation), which can be used if you just have a damaged install that won't boot. It reinstalls everything and puts the system back to almost the same as a fresh install, but doesn't wipe out the Registry or any user files, so all your applications should continue to work, and all your Favorites and stuff will work.
Installing WinXP over itself won't cause you to lose any of your files, but will make the OS a completely new install, so all your apps will need to be reinstalled.
The only time you should lose any files is when you tell Setup to format the drive.