Apple says that the original Apple TV is an "obsolete Apple product" and will not be updated to support security changes. Only those with the second generation of Apple TV or later will be able to access the iTunes store.
Windows XP or Vista users won't be able to use the latest version of iTunes, but older Windows computers can still use previous versions without support from Apple. Those who do so, however, won't be able to make new purchases or re-download previous purchases on that computer. If you're on on XP or Vista, you'll need to upgrade to Windows 7 or later to continue using iTunes normally.
The writing was already on the wall for these devices: Windows XP and Vista are no longer supported by Microsoft, and Apple has previously declared the original Apple TV obsolete.