I have a HP Pavilion 6500 that has recently started shutting down for no reason. I have had it completely apart, cleaned and inspected every part. Everything looks good and when I put it back together it worked fine for about 6 hours then shut down again with no warning or error message. When I attempt to restart, it comes on for about 2 seconds and shuts off again. (On battery, battery w/adapter and adapter only) Even when it has sat overnight or for a couple of days and should be completely cool. Someone in another thread mentioned processor protection in the BIOS. How do I check that? Any help would be GREATLY appreciated.

Ram can go bad, i would do a thorough test on the ram.... I use the ultimate Boot CD. It's free and it has some pretty good tests on the cd.
http://www.ultimatebootcd.com/

maybe It is strongly heated?

I have run the HP Health Check and everything comes back as good. It shut down again before I could run a directx diagnostic test on it. What do you mean by "Strongly Heated"? How do I check that?

I Russian... English not perfect)
How much it warm up after long work(you write 6h)?
You do not use it on a blanket, a mattress?
after what ths start?

I had it sitting on a table. Plenty of ventilation. Even after sitting unplugged for a day and a half it still shut itself off immediately. It is completely cooled off but still will not start.

Probably bad contact of the heat sink and hot unit (the processor, video the chip)
old thermal grease?