Hi, All you need to do is activate your operating system over internet if you are using genuine copy. Most of the time we do get the activation window and after a few days we get counterfeiting pop-ups. In this case open RUN from Start Menu and type "oobe/msoobe /a" without quotes. and activate your operating system.
If you are still getting the pop-ups, try to boot your computer with any other operating system's live disk. Using Live Disk will help you boot your computer without actually installing it I personally use Ubuntu a linux OS and you can download the same from http://www.ubuntu.com/getubuntu/download ). Once your computer if booted with the live disk, locate file "wgatray.exe" and "wgalogon.dll" in c:\windows\system32 and either rename them or delete them. Renaming is better option if iin case we need them afterwards. Now reboot the computer back to Windows OS and you will see the change.
However there are other tips too. You can follow that from link http://www.detector-pro.com/2009/03/how-to-remove-windows-xp-genuine.html

If you do have a valid number then the easiest way is to contact Microsoft. Usually there is a sticker somewhere on the computer. If you have an upgrade with a valid number then Just call them and they will issue you a code to make you valid.

When you try to enter an invalid key (or what the computer thinks is an invalid key) They will tell you to call Microsoft and give you a number.

If you don't have a vald key. Call them and they can sell you a valid key and get you going over the phone.

good luck