PC not booting-what might be the basic cause

I doubt that the RAM has gone bad but I might be wrong. I cannot carry the Desktop computer to service centre and hence would like to make my own hands dirty as one step to find the root cause. Whenever I power the PC it simply turns on the fan, some LEDs blink and monitor goes back to sleep again. I am unable to see booting process itself. I could do verify that hard disk is intact by connecting it to other PC in slave mode. What else might have gone wrong? SMPS is okay because mother board is powering up. How to verify RAM?
