Your PC specs and more a detailed description of your problem will go along ways toward getting you help
give a description of your computer like what windows are you using, cpu speed, amount of ram & does this restart happen before you get to windows or after getting into windows.
im running windows xp pro and 1gb of ram and 2.8 dual core and when its loadin the windows xp thing it restarts
when it does a restart next time, go into the bios/setup screen to pc monitoring or something like that to see what the cpu temp is. sounds like possible overheating as i've seen that on a friend's pc when his p4 cpu bracket broke causing the heatsink to tilt on 1 corner thereby causing the cpu to overheat.
I think thats your problem right there, i mean 70 Celsius is right around 160 degrees fahrenheit. I cant imagine that ever being normal. Is your heatsink getting power, fan spinning, heatsink properly attached w/ thermal paste?
the fan and everythin is goin yes and its always been that high i dunno why what should i do/ get to solve this problem?