I did this, and I like the result. Now if I can just relate to you enough to help. First off, there AIN'T no easy way. Next, it would be easier if you could do Linux, or if you are dual-booted with XP. I suspect older windows than XP would do. There is another thread here in these 7Forums at
http://windows7forums.com/windows-7-desktop-customization/2829-changing-startup-sound.html I didn't use it, but one poster says if you do not have dual-boot, you can do it in safe mode. Here, as best I remember is what I did.
I booted Xp -- Open "My Computer" -- My "7" is on another drive, so I opened that drive -- Navigate in "7" drive to: Windows> System32> imageres.dll -- I right-clicked and copy-pasted imageres to the desktop of XP. At this point others use ResHacker, but I did some reviews on it, and instead I downloaded and installed
XN Resource Editor I used that to open the Imageres file because Windows can't/won't open it. In 7 the file that is the sound is 5080 and that is what you have to change to change the sound. Now, you must prepare whatever sound you want, rename it 5080.wave It must be a wave sound. Delete that wave that is in the original file and insert the new one you made. After I renamed mine, I tested one time to be sure it played.
Now this is where I used linux. I shut everything down,put a
Knoppix cd in and booted from it. Then, running Knoppix, i entered W7, navigated back to imageres .dll As I remember, at this point, I renamed that file to imajeres and left it in place, then I copied the new modified imageres into the same folder. Thats it! Mine works!
Everyone who has done this has done something different. you will just have to read and work your way through. Maybe you have a techie friend who does Linux.
CAUTION!! to all who try this! I don't guarantee nothin! If you are not capable to repair any harm you can do inside Windows\ System32 , better stay out of Windows files.