New motherboard installation

What is your old computer Name (Brand) that you are removing Windows 7 from to install on new computer?

I agree with the majority that a clean install is the way to go. If you have a retail OS then no problems.
If you have an OEM OS then it strictly is tied to the motherboard and MS are not obliged to activate it on a new motherboard. This is why OEM OSs are cheaper than retail. Like others have said give MS a try otherwise buy another OEM W7.
By the way your HDD/SSD has noting to do with the OS authentication. When you want to change these just do an image restore to the new one.

Bradner said:

It is custom build. There isnt a known brand!

If it is a Custom Build, there is a good chance the Windows is a Retail Version which means you can have a new motherboard with no problem. When you installed Windows in the machine you have, did you buy Windows at a store?

Bradner said:

In all matters I will do a fresh install of windows...I will contact with windows for the activation process. All the difficulty might be there!

When you put the Product Key into Windows, it either accepts what you put in or gives you a phone number to call. Don't call until it prompts you to. You might not need to call.

Click on your start button and right click on "Computer" and left click on "Properties". At the bottom of the screen near the left you will see "Product ID". If it has an OEM in the number, then you can not change the motherboard without getting involved with a call to Microsoft.