Comp keeps rebooting!!!?

Ok i have reformated and done everything i can think off!!!! I put windows 98 in aqnd can get into safe mode but there is nothing that fixes it and then i formated again and put xp on and now i can not get into safe mode it just reboots. is it the mother board? or power suply? and if you think it is how do i go about checking it? the ram is good bec i took it out and it will not even atemt to boot up without it!!!