Fortunately, Avid announced today that Sibelius 7 is fully supported on Windows 8. Previous versions of Sibelius should work as well, with some caveats. According to Avid’s Sam Butler:
I’m pleased to announce that Sibelius 7, Sibelius 7 First and their respective sound libraries run well on Windows 8 and are fully supported.
Although we haven’t qualified Sibelius 6, we couldn’t find any problems during our limited testing.
The latest versions of PhotoScore and AudioScore are fully supported, as well as the latest versions of Auralia and Musition.
This page at the official Sibelius help center gives full details.
Image attribution: Used with permission from Microsoft.