To reset the password of your user: insert install disc 1 that came with that Mac to the optical tray, and restart the Mac holding down letter C at startup.
At the installation screen, select language, then from the menus Intaller (or Utilities) > Reset Password, and in that dialog select the account you want to reset the password for. After reseting it, restart the Mac normally.
If that was all you want the reinstall for, stop there. No install needed for that.
After booting first time after the password change, open Keychain Access (/Applications/Utilities) and select from menus Keychain Access > Keychain First Aid > select user, and Repair.
If you have other reasons for reinstalling the OS, boot similarly, with C down, to the install disc 1 again, after making sure you have a backup of everything you want to keep.
From the installer menu, continue the installation, and select install type (archive and install, clean install, or upgrade install that will not be an option this time), and select the options you need and install. Prepare 20-60 minutes for finishing the installation.