Your desktop will change back to a solid color till you fix your regestration problem. If your case has the key on the side/back check and see if it matches the key that was used when it was installed. If a retail version was used to re-install XP (I am assuming you had a valid version on this pc prior to a re-install) your OEM version (Dell,Gatway,IBM,Sony,,,) key will/did not wwork. on a retail ( green box from Walmart, Bestbuy, Comp USA, ....) disk (burnt copy or not!!) The installer probably just duplicated the retail key. I always back up my disks to protect the orignal from dammage and have not had any problems with reinstalls. You will have to change the key to the correct one or obtain an OEM disk to get back up and running. If there is not a key on your case you will more than likley have to buy another copy of XP/w/key and reinstall all over again. Unless you can find the orignal disk that went with your PC.
If your key does match somone else has used your key on another PC.
In which case you also wll have to get another XP/w/key!!