I think you mean Riff?
http://www.microsoft....sonalize/default.mspx is a good place to start, and then here:
http://www.microsoft....rsonalize/sounds.mspx
"Sounds have to be in the .wav format"
If your sound is in a MP3 format, use a program like Audacity to save the file as a ".wav"
The following is from here:
http://kb.iu.edu/data/ajgs.html
Windows XP
From the Start menu, select either Control Panel, or Settings and then Control Panel.
Click the Sounds and Audio Devices icon, and then click the Sounds tab.
Under "Program Events", select Start Windows. Click Browse to select a .wav file. If you don't want Windows to play any sound during startup, select (None) from the drop-down menu.
All the best,
Spare Ed