Sudden reboots..I took care of a similar problem a while ago.
The way u discripe how things are unfolding..let me see.
sudden reboots and difficulties with the processor, HardDisc damaged etc. I'm not saying I know 100% what's the problem, but I could be a case of leaking capacitators. I wouldn't harm to at least exclude it though.
I would advise u to open up ur case and check-out the capacitators on the board. Look carefully at them and see if there are any of them with black/brownish stuff on top of 'em or at the base (where they are connected to the motherboard)coming out. Also try to determine wether there are any bulged capacitators present on the board.
Here's an article has some picturs of what a leaking capacitator looks like. Don't all the technical talk in the article, just look at the pic's
Leaking capacitator can cause un-stable behaviour of your system, damage parts such as hard-disk's, CPU, soundcards and videocarts and ultimately lead to a destroyed motherboard (if you look at the pictures you can figure out why)
A different cause may be faulty RAM, this also happen occasionally and also causes unpredictable behavious of ur PC