Microsoft will drop support for Windows XP on April 8, 2014.
While there are sources that state the Microsoft downloads will still be online afterwards for people to update their machines, there are no guarantees of this. Microsoft is only obligated to host these files until the end of support. Windows Update for Windows 95/98 and NT were pulled offline a few years ago and no longer functions.
In the future, if you need a copy of Windows XP Service Pack 3 (SP3) and the file is no longer available online at its current location (http://download.microsoft.com/download/d/3/0/d30e32d8-418a-469d-b600-f32ce3edf42d/WindowsXP-KB936929-SP3-x86-ENU.exe) you can always download it from the Internet Archive, a US government non-profit organization dedicated to archiving the web since 1996.
You’ll simply need to click the following link, which will give you the option to pick a date from which you’d like to download the service pack.
Service Pack 3 was released on May 6, 2008. The earliest archive date available is July 25, 2008 (http://download.microsoft.com/download/d/3/0/d30e32d8-418a-469d-b600-f32ce3edf42d/WindowsXP-KB936929-SP3-x86-ENU.exe)