Boot problem. Hi.. m havin a new dxktop pc..with asus p5n-mx.. duel core 2ghz.nd 1.25gb ram with

Question

Hi.. m havin a new dxktop pc..with asus p5n-mx.. duel core 2ghz.nd 1.25gb ram with inbuielt gforce chipset.... so my problem is..after swetching on my pc the screen remains black...nothin is goin on.. and after few mins its getin off..... but i went to asus my board is ok.. ram is also fine.. nd i hav put my cpu into my friends board..tht also fine.. means all parts r fine..but after getin together its not runing..... i have used it for 1 month.. suddenly this problem came on... plz tel me what to do.............