Well, if you have bought a legal version of Windows XP Home, try using this program while logged in...
I also have a legit copy of Windows XP Pro, but after reformatting and reinstalling Windows for the millionth time, it gave me an activation error everytime...
So i began to look for something to stop that. I found something called "WPA Kill".
This will disable WPA
* so you won't have that annoying message popping up everytime you start Windows.
(Don't worry, this is clean, no spyware/virusses

)
Description:
Windows 2003 & XP Anti Product Activation Patch
Download:
WPA_Kill.zip
If Microsoft fails to support their customers the proper way, we have to switch to alternatives ourselves...
*Windows Product Activation