Hi there and welcome to PC Help Forum.

First thing to check, if you have a quick look inside your case with the power on, can you see any fans that have stopped spinning?

First thing is first. Always check this before you you mess with any software because somep people seem to overlook this sometimes cause I did it. Check to make sure the fan on your fan is working because what happens is that when your processor gets too hot, your computer will reboot itself. So check that. If thats working properly then try flashing your bios if you have another computer available and see what happens. Also, what do you have loaded on there as far as antivirus, spyware, firewall etc.?

yes, all fans are spinning ok. is strange but, it reboots not always in the same time. i tried safe mode again, but when it started to show the list loading files, it rebooted.

i did not installed any hardware or software.

In the safe-mode menu, please select "Last known good configuration"

"last good known configuration" reboots too. what is to flash the bios?

i inserted the win xp cd, booted from there and load recovery. it started to analize some things, and suddenly it reboted.