Supervai,

The computer has diagnostic indicators. Does your computer have any of the below diags showing? If there is only a light on the motherboard then, the motherboard may be bad and  will need to be replaced. To replace the power button you would need to replace the front bezel. I do not think the problem is the power button. Also, you may need to replace the power supply. If you remove the memory and if you have a discrete video card remove it as well. Try turning on the system to see if you hear any beep codes from the system.

If you need to replace parts you can contact Dell Spare Parts at 1-877-717-3355 or click the link below.

Dell Spare Parts

Power Button Light

The Vostro™ 200 power button light (bicolor LED) located on the front of the computer illuminates and blinks or remains solid to indicate different states:

	Off	The computer is either turned off or is not receiving power.	Reseat the power cable in the power connector on the back of the computer and the electrical outlet. If the computer is plugged into a power strip, ensure the power strip is plugged into an electrical outlet and is turned on. Also, bypass power protection devices, power strips, and power extension cables to verify the computer turns on properly. Ensure the electrical outlet is working by testing it with another device, such as a lamp.
	Blinking amber	The computer is receiving electrical power, and power supplied by the power supply is normal. A device might be malfunctioning or incorrectly installed.	Remove and then reinstall the memory modules. Remove and then reinstall any cards. Remove and then reinstall the graphics card, if applicable.
	Steady amber	This is a system fault error condition, including the power supply. Only the +5VSB rail on the power supply is working correctly.	Ensure all power cables are securely connected to the system board. Ensure the main power cable and front panel cable are securely connected to the system board. NOTE: If the system was abnormally turned off, you need to disconnect and plug in the AC power cord. Then turn on the system. Otherwise, you may see an incorrect power light state.
	Blinking blue	The computer is in Standby mode. Press a key on the keyboard, move the mouse, or press the power button to resume normal operation.	If the power light is blue and the computer is not responding, do the following: Ensure the display is connected and turned on. If the display is connected and turned on, listen for a beep code.
	Steady blue	The system is fully functional and in the On state.	If the computer is not responding, do the following: Ensure the display is connected and turned on. If the display is connected and turned on, listen for a beep code.

System Beep Codes

The Vostro™ 200 system can emit a series of beeps during start-up if the display cannot show errors or problems. These series of beeps, called beep codes, identify various problems. One possible beep code consists of repetitive three short beeps. This beep code indicates the computer encountered a possible system board failure.

The series of beeps has a 3-second delay before it repeats. The series and delays continually loop until you press the power button.

I had this problem for almost 3 years on-and-off. At first, I changed to a new power supply, it went on well for about 2 years and then the problem came back intermittantly.

I found out that if I removed the front bezel cover, and manually push the ON-Switch, it will work. The cause was due to the bezel ON-button not pushing in enough onto the inner ON-Switch.

Lately it can't be powered up easily even without the bezel cover and also by manually pushing  the inner ON-switch. I changed another new power supply unit but the problem persisted. I am inferring there is something going on between the On-Switch and the circuitry in the motherboard.

To by pass this problem; just go to BIOS setup, and set the Power Management for AC Recovery to "On". The next time you power up PC via the main switch, the system will on by itself without the need to press the ON-Button nor the ON-Switch. 

I won't want to waste my time tinkering further; I rather spend more time using the PC.

LoL. Hope this helps till a permanent solution is found.