The Microsoft License for use of Windows has always been limited to allowing installation on only a single machine (and that excludes having the same copy installed on a laptop as well as a desktop machine: only MS Office is licensed for the combination). Microsoft believes that this has been subject to much casual abuse. WPA is a means of ensuring that a single copy is not installed on more than a single machine.
So, within the first 30 days after installing Windows XP, you must get the system ‘activated’ if you are to be able to go on using it. This involves the computer dialing in and giving some information about the hardware on which Windows is installed, receiving in return a release code which will be recorded on the system. More is said below about OEM copies provided preinstalled on a new computer
At subsequent boots, Windows checks to see that it is still running on hardware that it can recognise as being the same. If it does not match well enough, you will be unable to do more than backup files until you call Microsoft to explain — for example, that the old machine broke down and had to be rebuilt — and get a new release code. In other words, you will need to reactivate your installation of windows.
What hardware gets checked?
The WPA system checks ten categories of hardware and records votes from each one or rather windows votes from each one
Listed below are the hardware that windows checks:
1. Display Adapter
2. SCSI Adapter
3. IDE Adapter (effectively the motherboard)
4. Network Adapter (NIC) and its MAC Address
5. RAM Amount Range (i.e., 0-64mb, 64-128mb, etc.)
6. Processor Type
7. Processor Serial Number
8. Hard Drive Device
9. Hard Drive Volume Serial Number (VSN)
10. CD-ROM / CD-RW / DVD-ROM
This is true from windows XP forward… so this is also true for windows 7.