According to an article at Neowin, on April 12th Microsoft plans to force SP2 onto all Windows XP and Windows XP SP1 machines. Prior to that time, users have been able to say no to SP2 because they have had the option of temporarily blocking the update from being downloaded to their machines through the Automatic or Windows Update mechanisms. But this is not going to be the case any more after April 12th, and SP2 will be getting delivered whether you like it or not.
"The intent of this alert is to provide you with a reminder about the upcoming deadline around the date on which Automatic Update (AU) and Windows Update (WU) will deliver Windows XP SP2 regardless of the presence of the blocking mechanism."