System Crashes And Reboots Often And W/o Warning/reason?

my PC has started rebooting without any seeming rhyme or reason -- and each time it restarts the disc check finds unhappy things.at first i thought the crashes were because of system overload -- too many programs running and/or tasks i was having it do all at once. with no applications running - and it'll blip out and reboot all on its own. more than once.i know there isn't a virus; i run norton and i also went digging, just to be sure. i also finally had it checked out; they couldn't find anything specific. i've stripped about all i can think of from my start-up and have removed software i like but don't have to have, trying to make it easier on the computer.the 'crash' doesn't give any warnings or errors. my computer simply does a restart.