hi, i have a big problem wif booting my system

when i turn on the system (im running winxppro), usually there would be no signal to the monitor. but as i keep turning on and off the system, the condition "improves", and it'll turn on, but shut down(no signal to monitor) when it starts to boot windows. as it improves, it shuts down during the login screen and sometimes when i do get into my desktop. and then if im lucky, the condition stabilizes. and then at some point of time, the condition would start deteriorating.

if it helps to say. that when there's no signal and i hafta turn off the power and on again, sometimes i hafta try it a few times before there's a signal. during the "failed" attempts, my cd-rom, power and activity lights stay full on. and sometimes, only my fans can be heard running. if there's the signal, the cd-rom light goes off.

im not sure what the problem is. its not the power supply, i've tried switching to a spare one. pls help!