Computer will not boot

that can be meaning if the processor is Not Supported with your motherboard,or the New Processor is not in good condition (can be Over Heat or damage).
Processor over heat or damage can be because the HEAT SINK (white or grey pasta at the top of processor) is not enaugh to cooling the processor or,the fan Proseccor is Dirty.

Duo core motherboards will not support the bus speed or the power output to power on that processor. You need to upgrade the motherboard with the new processor.

Power Supply. Weak voltage power rail.

Has enough power to light LED lights, maybe spin fans, (Or spin a few times, and stop), but not enough power to turn the Processor on.

1) ALL of the LED lights combined use less than 1 Watt of power.

2) EACH fan uses 2 to 3 Watts of power.

3) A typical Processor can use 51 to 125 Watts of power.
Depends on what Processor it is.

Acer Aspire M1641-U1521A?
Intel Pentium Dual Core E2180 (2.0GHz)

http://en.wikipedia.org/wiki/List_of_Intel_Pentium_Dual-Core_microprocessors#.22Allendale.22.2C_.22Conroe.22_.2865_nm.29

Can use up to 65 Watts. (TDP)

The Power Supply used is a 250 Watt unit. Acer isn't known to use high quality power supply's. It's a generic low quality Power Supply.

[ However in all fairness to Acer, if the inside of the computer, AND the inside of the Power Supply, isn't cleaned out on a regular basis as needed, this can cause Power Supply failure also.

Use a can of compressed air for computers, with the plastic 'straw' attached. Hold onto the can with one hand, use the other hand to hold onto the straw, shoot air through the ventilation holes of the Power Supply's case. (Computer unplugged from power)

Come around to the back of the computer, use the air to clean the Power Supply's fan blades, center hub, and surrounding cage. (Shroud)

When the cooling components for the Power Supply, (Fan and Heatsinks used inside), become clogged with dust, dirt, etc., the cooling capacity drops tremendously.

The Power Supply overheats.
Heat = Wasted Energy

The Power Supply strains to keep up with the call for power, but eventually cannot, and fails.
Components inside the Power Supply fail ]

Replace the Power Supply.

Did you install the CD disk that comes with the motherboard? It's the motherboard DRIVER'S disk.

when you boot your computer open up the bios menu, i forget which tab it is under but there should be an option called S.M.A.R.T fan or SCSI if it is disabled enable it this should fix it, if you do not have this option or if it doesnt work this is a link to a fan control program called speedfan http://www.almico.com/sfdownload.php

Check the heat sink connected on your processor and its cooling fan is working or not

Did you know that you would need to have a more powerful power supply?
Dual core cpu's need a minimum of 600 watt power supply.

You need to have a PSU greater than 450watts. that could be the problem..

Nice system, what kind of cooling do you have in there? How many fans etc...

anniegms, I had the same problem with a new Asus board running a Dual Core Duo Quad processor. The boards BIOS was not updated to handle the Dual Core Duo yet. I had to switch processors to a P4. Use a different PC to make a BIOS update disk. Update the BIOS and switch processors back. But I had all the same symptoms of yours. However I did/do have a PCI-E XFX Fatality (Nvida) based graphics card. I would check or swap processors, it sounds like the board is waking up but not the processor. Just my thoughts.... PCdok