Reboots can be caused by something overheating like CPU (when was last time it dusted out and new thermal paste applied between CPU and HSF?), by failing RAM, or failing HDD. You can use memtest86 to test the RAM. You can boot into BIOS and let idle for about 10 minutes while watching the temperature reported on the CPU. There is software you can download and install to watch CPU temps, but need to adjust it's reported value on first boot to about room temperature plus 10 F or 5C. So if room temperature is 75 and it reports the CPU at 100F, then figure the software reading is about 10-15 degrees too warm on it's report. Some software, like I use in linux, allows the "offset" to be adjusted for that.
My computer's been spontaneously rebooting on an almost daily basis recently. Where's the log file that will hopefully show me why it's happening? Do I need any special tools to interpret the log?
(If it's my RAM, I'm expecting to replace it anyway today.)