According to a support document recently published by Apple, the company will soon be introducing security changes that will prevent Windows XP or Vista PCs and first generation Apple TVs from using the iTunes Store. The changes will be implemented starting May 25, so if you're still using PCs running those operating systems or you've been holding onto your old Apple TV until you've gotten all the use out of it you can, now may be the time to upgrade if you want to continue using the latest version of iTunes.
The reason for the upgrade is essentially to cycle out tech considered to be out of date. Windows XP and Vista are no longer supported by Microsoft at all, and Apple TV (1st generation) is considered to be an "obsolete" device by Apple itself (meaning it will not be updated to support iTunes — or any other — security or software changes).
If you absolutely have to hang on to your old PC for the time being, you can still continue to use previous versions of iTunes on both Windows XP and Vista without support or further updates from Apple. That being said, you won't be able to make any new purchases from the iTunes Store (so no more new movies, albums or TV shows), nor will you be able to re-download previous purchases from other devices on that particular computer. Once May 25 rolls around, you'll need to use at least Windows 7 in order to access the latest version of iTunes or purchase any new content from the iTunes Store. The same goes for Apple TV — after May 25, only second generation Apple TVs or later will be able to run iTunes.
Go forth and prep accordingly!
If you have any questions regarding Apple's security changes, sound off in the comments below.