Go get all the updates from XP and get it up to SP3. Also, if there is norton or other that has expired or the date/time is wrong, this happens. Fix? Get norton/other updated or removed and set the date/time.
As this post tells so little my reply is based only on the clues given.
I have a laptop, running Win XP. I can connect to the router and it shows I have a strong connection signal, but when I try to open any ISP (Google Chrome, MSN, Outlook, etc) it comes up saying web page not available. All my other computers connect and go online just fine using this same router.