Apple yesterday published a new support document noting that security changes being implemented on May 25 will prevent the first-generation Apple TV and PCs running Windows XP or Vista from using the iTunes Store. Apple has also begun emailing users with active first-generation Apple TVs to warn them of the upcoming change. 

Beginning on 2018-05-25, Apple will introduce security changes that prevent Apple TV (1st generation) from using the iTunes Store. This device is an obsolete Apple product and will not be updated to support these security changes.

Apple notes that second-generation and later Apple TV models will continue to work with the iTunes Store. 

The first-generation Apple TV based on a traditional hard drive was introduced back in 2007 and was supplanted by the much smaller flash-based second-generation model in late 2010. The first-generation model was officially declared obsolete by Apple in late 2015. 

As for PC users, Apple notes that machines running Windows XP or Vista are no longer supported by Microsoft and are unable to use the latest versions of iTunes, as iTunes 12 requires Windows 7 or later. With the upcoming changes, users running these older versions of Windows and iTunes won't be able to make new purchases or redownload previous purchases from the iTunes Store. 

Source: macrumors