Pc doesn't boot up even all fans are running but sometimes works perfectly.

[naveenkills]

My Pc doesn't Boot up some times even the processor fan and psu fan are running but some times it works perfectly there is no such problem .. and I recently added a new psu and a hard drive ..
My PC specs are :-

Processor :- AMD sempron 140
HDD :- WD Green 500 GB
RAM :- 3gb (2 GB Hynix PC3 10600U and 1 GB Kingston PC3 10600U) 1333mhz each.
PSU :- Enter 500 W power supply.
Motherboard :- ASUS M5A78L-M LX V2
GPU :- ASUS NVIDIA GT 610 2GB

This pc is about 7 - 8 years old but I never encountered such problem until I tried some RAMs coz I wanted to upgrade my 1 GB ram to 2 GB but none worked .. please suggest me a 2GB RAM and What could Be the reason for this problem .. I have tried some methods like .. Removing the CMOS battery for few minutes and removing all cables for few minutes etc .. but its the same some days later ..
Hope You guys can understand my problem and help me any kind of help will be appreciated..
Thank You..

 

[rocktalkrock]

Is your motherboard updated to the latest BIOS? Many updates were issued to "Improve system stability". Your Enter brand power supply is not a very good quality as well.