Hey your saying your pc Restarts on its own if it does you may 3 problems
If your lucky you have a.....
or deleted something
But if your not Lucky you may-have a....
i don't know if you ever seen one well they these like lil batteries on them
and when those start to leak computer restart it self
But alot of these new pc's come with backup install built in so press
f11 or f12 forgot lol
at the binning of boot if you don't i could upload
Windows xp Pro
just let me know