PC not turning on

I am using a PC from more than 3yrs...which have
Pentium4 HTT processor,
512mb elixir RAM,
Intel D102gcc2 mother board,
LG DVD RW drive,
160gb hard disk...

Some days b4 my PC suddenly strtd restartng again n again n it happend upto 3times...i switchd off the power directly n aftr an hour i switchd it on n normally it turnd on...bt aftr 5min it went off...aftr tat im nt able 2 turn it on..

I chkd my mthr board...green LED on it z lit..so no mother board problm..
I tried 2 short pins on board tat goes 2 pwr buttn bt gt nthng...no sound of fan r hard disk r nythng...i thnk somthng wrng wid pwr buttn coz i hav testd SMPS using a testr...the pwr supply 2 SMPS is wrkng 5n...n no leakd r bulged capacitors in SMPS...

Im looking for a solution r atleast a procedure 2 diagonize where da problm lies....So plz kindly help me, dose who hav solutn

Thanks & Regards
Sai Krishna