I replaced parts in this computer and now windows is asking me to activate it but it was working fine before i replaced parts, and the product key off the back of the case doesnt work. how do i get windows activated

When the Windows operating system is installed on a computer, it assigns hash values to major hardware components inside the computer.

Some of the major hardware components such as,

1) Processor

2) Harddrive

3) Motherboard.
More specifically the motherboard chipset, and BIOS program, plus the version of the BIOS program.

4) GPU. (Either the GPU used for Integrated Graphics, or installed on a graphics card)

[Northbridge/Southbridge chipset, is the motherboard chipset.

Chip and Chipset are slang terms for I.C.
Integrated Circuit.

http://en.wikipedia.org/wiki/Integrated_Circuit

This is a motherboard diagram, showing the relation of the Northbridge, and Southbridge chipset,

http://en.wikipedia.org/wiki/File:Motherboard_diagram.svg

{CPU stands for Central Processing Unit. Another name is Microprocessor, or simply Processor for short}

The Northbridge chipset handles the high speed capabilities of the computer.

Such as the Processor, Ram Memory, and High speed graphics.
AGP and PCI Express.

A graphics card inserted into a PCI expansion slot, is not high speed graphics.
Integrated Graphics is not high speed graphics.


The Southbridge chipset handles the slower capabilities of the computer.
Anything inserted into a PCI expansion slot.
IDE, or SATA harddrives.
USB
Ethernet, and so on.

My sources state the eMachines T6420, uses an MSI ms-7207 motherboard. (K8NGM2-L)

(Haven't had one come to my shop to verify this)

http://www.newegg.com/product/product.aspx?Item=N82E16813130041

This MSI motherboard uses an Nvidia GeForce 6100, Northbridge chip, and an Nvidia nForce 410, Southbridge chip.

GPU:
Graphics Processing Unit,

http://en.wikipedia.org/wiki/GPU

Apologizes for the long dissertation, but wanted to explain the motherboard chipset, and GPU, in case you were not aware ]

Change any two of these components, and Windows wants to be activated again.

Changing the motherboard alone, could also do it.

This implementation was brought about by Microsoft, to deter thievery of their software, (Operating System)

If there is an Activation message listed, I suggest clicking on it.
There will be three options given.

I suggest clicking on Call Microsoft.
(Toll free number)

I myself like to speak to a person, rather than activate over the internet, possibly run into trouble, and not have a human being to help sort it out.

[NOTE from me.
If you get a computer voice, after it has asked a question, make a sound like a buzzing noise.
After you do this three times, you will be referred to a human being]

After you click on Call Microsoft, there will be a series of boxes on your screen. each box has a series of letter/numbers in it.

When you call the nice person to activate, you will be asked what you wish to do.
Activate.

You will be asked for those letter/numbers, one box at a time.

Suggest you speak clearly, and make SURE they repeat them back to you, as you give them, one box at a time.

When you have finished, they will state "Hold On".
(Or something to this effect)

The person will give you back a series of letter/numbers, which you will type in the blank boxes on your screen.
One box at a time.

ENSURE that you understand very clearly. If you wish, they will repeat.

Once you have filled all 6 boxes, there will be silence.
You are expected to know that you have to go to the bottom of the screen, and click on Activate.

Once you have done this, you are finished.
Your genuine copy of Windows is now activated again.

You have a new Product Key, and the one that came with the computer is no longer viable.