unidentified problem

i have a P4 2.2Ghz compaq system with 1Gb RAM, Vista ultimate & XP home dual boot system. the system shuts down unexpectedly & restarts after a few moments only to shut down & restart repeatedly. if i switch on after sometime it runs for sometime and same thing recurs again. after i changed to a higher capacity SMPS, the PC ran well for a few days but again the problem has restarted. it occurs both with XP & vista. the cooling fans seem to be running well & i have tried running the PC with all other hardware like DVD drive, modem, etc disconnected but same problem persists.
please help.
ramkumar