Probable cold boot problem, or HELP me find out what !

Yo,

so i just installed my new mobo, an ASUS P5G41T M-LX3 ,and an nvidia gt520 1GB DDR3.

and there was no problem for about 3days, and now when i try to start the pc, the fans(cpu and psu) works.mobo LED stays on ,but the keyboard and mouse show no sign of power.

after switching off the supply and starting it again for 4-5 times ,it will start,sometimes stays on ,sometimes freezes and crashes.

i replaced the cmos bat ,updated the bios to the most recent version, still the same problem.

specs .
pro: intel pentium 4 506,2.66 GHz (yes,very old and weak) (im testing this ccpu, my original cpu is intel pentium dual core e5200)
2GB DDR3 ram
500 GB WD HDD
a 450 watt PSU(local made)


i just bought the mobo, so i really hope there is no problem with it .


if anyone has got any helpful suggestion ,plz help .

ty!