Need more info...
What are you doing when it restarts?
If it restarts doing something specific does it do it every time?
How long is the computer on when it happens?
Do you get a BSOD?
What error message is in the blue screen?
What does the bios say your cpu temp is?
About the hard disk thrashing...
When's the last time you defragged?
What software are you running when it thrashes?
Some easy checks...
Make sure your power supply connections are good.
be sure your power cord plugged in all the way at both ends.
Check your power bar plug and be sure it's in all the way.
Try a different power bar, or plug directly into the wall.
Try a different power cord.
Try a different keyboard.
--->It ain't better if it don't work<---