Just because the popup is gone, that doesn't mean the problem is gone. If this was a genuine WinXP message, you will not be able to receive security updates for Windows anymore.
You need to contact Microsoft support to find out what the deal is. FYI, just because your computer came with WinXP installed and you have a disc, that does not mean that it is necessarily genuine. Many dishonest dealers have sold computers with pirated copies of WinXP software. I'm not saying that this is the case with you, but you should not discount the possibility without talking to Microsoft first.