A Service Pack is a bundle which provides all the patches and fixes Windows needs to bring your computer up-to-date. However, Microsoft doesn’t let its service packs install on illegitimate copies of Windows XP. If your copy of Windows XP isn’t installed legally, the service pack leaves this message when you try to install it: The product key used to install Windows is invalid. If you see that message, contact the person who sold you Windows XP. Microsoft has released 3 Service Packs. To find out which one you own, click on the "Start" menu, right click on the "My Computer" icon and choose "Properties". In the "System Properties" dialog box that opens up, you can see the Service Pack mentioned. Having Service Pack 2 installed is like receiving an updated version of Windows XP with new features, all for free. Even after you install Service Pack 2, you still need to use Windows Update. That keeps you updated with fixes discovered after Microsoft released Service Pack 2.