HELP my computer randomly restarts :(?

i have an advent 3415 tower unit with a pent IV processor i kno not top o the range but it does the job it has a 512 7300 gt graphics card i have no idea wats rong with it ive dun virus scans ive cleaned and defregged the registry, i also regularly maintain files and such so tht i can save space but as of yet it still restarts i have added new hardware recently and i have removed tht and it still restarts. i kno this is a very long question but i REALLLY need help this is drivin me mad