Pc will turn on but not boot!

yelp... similar problem (writein this message on wii internet) my computer turns on! for 1second all theater capslock, scroll lock, and numlock come on and go off a second later, then nothing happens, screen wont show anthing and the keyboard wont light up! all the fans are on! it has been repaired twice in the 5years we have had it! if i can`t repair it now bymyself its getting scrapped lol... shame becasuse its lasted a long time!!

its got windows xp home,

i have read Similar problems and most say to replace the rom!!!!! or the mobo! if that is the case!! how do i know what im looking at??? thanx for your help!!!!!