In Control Panel select sounds & audio devices. On the Sounds tab the first drop down box shows you the Sound Scheme in use. You could at this point select windows default and everything will revert to original settings.
If you are only missing the windows start up tune in the second drop down box marked program events scrollto "Start Windows" in the alphabetical list and then click on it to highlight the words. Below should appear in the Sounds drop down box "Windows XP Startup.wav" My guess is that it actually says NONE. Anyway I suggest you change it and change it back again.
If that doesn`t work check to see by searching your computer if the file is even there - it may have been deleted or moved by mistake.