I had the same problem but it turned back on right away.
I flashed ( this happen 4 months ago) bios and that fix it.
4 days ago i flashed new bios( had new troubles) and now it turns itself back on again upon shutdown just like before. so i did a clean install fsdk /format all went but it stops at setup!!
Please see my other post (most recent) i need help!
i'm on webtvfor 4 days trying to get help(see my other post)
I'm at email@example.com if anyone has any suggestions,I NEED MY PC BACK!
" Soon My Friend , Soon...."