Will Windows XP really no longer be on sale after June 30?
Sorry for the double negative, but no. All this means is that Microsoft will stop selling the OS. Finding a computer with XP preinstalled will likely be very difficult, as well. However, you'll still be able to find copies for the foreseeable future, and likely the unforeseeable one, too. See for yourself: You can find copies of just about any Microsoft product, including ancient versions of Windows and even MS-DOS, by simply searching online. But hang on to your current copy of Windows XP. You may need it down the road if you don't want to move to Vista.
Will I be forced to upgrade to Vista soon?
No. But it will get harder and harder not to, especially since new software and peripherals are likely to stop working with XP. That could take years.
Will my XP machine stop working in June?
No, but Microsoft will stop releasing non-security software updates to the masses on April 14, 2009. But let's be clear: XP will continue to "work" even after this point.
Will Microsoft shut off product activation for XP after June?
No, that would be crazy. While no one has said this will happen, it's conceivable that Windows could shut down product activation for XP at some point. But that would only happen after XP reaches its end-of-support term (when all support plans expire). The good news for you: That happens on April 8, 2014, which should be plenty of time to get the kinks worked out of Vista-or switch to a Mac, Linux, or anything else. Bottom line: Your copy of XP will work, totally legally, for at least six more years.
What about after 2014?
Can I install XP on a PC that has Vista already on it?Well, that's unclear. But it's possible XP will stop being installable at that point. Microsoft's official policy is that these dates have "no affect [sic] on how long you can use a product," which may imply product activation will work forever
More @ NeowinYes. You can delete any partition with any OS on it (and reformat it) during the installation of XP. If you're feeling brave, you can even run both OSes at the same time by setting up a dual-boot machine.