Remove the line for the old OS in [operating systems] by going to Start/Run, and typing C:\BOOT.INI Be sure 'default=' points to the new OS
(or) In XP, press WinKey-Break,(or start/run 'sysdm.cpl'), Advanced Tab, "Start Up and Recovery" area, Settings button, then edit the old line out with the edit button. Reboot, and delete the files for the old OS. Don't format C:, you will need some stuff in the C: root.