check your memory like anthony said, a stick at a time.
If it continues, check for power issues. If you have a lot of hard drives/cd rom drives what have you, try unplugging a couple that are needed to test what the issue is. If after a while you do the same thing that you were doing with it rebooting and it doesn't, then it's a power supply.
If it STILL does the same stuff, try the heat thing by taking off your case side and putting a fan on it directly to see how much it affects it. If none of those things solve crap, then start thinking hard drive. If you replace the HD and it does the same shit yet again, then your mobo is just going down the tubes but usually random reboots are the above mentioned.
T.c.