Hey, Rob311 you do you fix...
How to Change Windows XP product key (serial number, volume license)
Sometimes Windows XP complains that your product code did not pass the "genuine Windows validation" like so:
This can happen for a few reasons. The most common being your product key (either knowingly or not) ended up on the internet. Once your computer "phones home" back to Microsoft it checks your key against known compromised keys. If your key is on the list, Bam!, Nag screens. Microsoft understands these type of things happen and will normally sell you a valid key code at a discounted rate or even free in some cases where you can prove that you own the key.
Once you have a valid key code, it's pretty easy to fix. In fact Microsoft has 2 methods to fix it and they even wrote up an article on how to do so here: http://support.microsoft.com/kb/328874
Method A) Involved more steps but seemed pretty easy for anyone to follow.
Method B) Automated the whole process of Method A, but I can see a begginer getting lost pretty quickly.
With either path you choose, after a reboot, you'll notice your Desktop is back to normal. If it's not, either your new code is invalid or you missed a step.