Display

Hello,
my name is imran.
one day when i was using my P4, sudddenly it hanged.i restarted it and monitor was showing no display no message.
i ejected ram(256), heard beep twice, inserted the ram and restarted the pc, it didn't work.
i ejectedmodem and restarted the pc, it didn't work.
But somtimes when computer started well then another thing happened.
when i start a program from a shortcut on desktop computer shows display problem, but when i start a program from a drive in my computer it ran smoothly.
2nd when i let the CD ROM opened, computer ran smoothly. so i plugged out the CD ROM and floppy drive.
i used to clean the ram with new cotton cloth.
But still i have the problem that sometimes PC runs smoothly and sometimes it hangs or resarted.
Can it be a problem with power supply.