I recently upgraded to Windows 8, and installed Visual C# 2010 Express along with XNA Game Studio 4 (like I had on Windows 7).
When compiling my project, I had an issue with spritefonts. Basically they became blurrier and not at all optimal. Here's a screenshot of how it looks (the left is the game compiled on Win7, the right is compiled on Win8).
https://i.stack.imgur.com/92dj7.jpg
If I manually replace the Win8 compiled (.xnb) spritefonts in the Content folder with the Win7 ones, the problem is solved. However, I don't want to spend a lot of time manually replacing content each time I run the game.
I had the same issue when I upgraded to Windows 8 and a simple workaround I found was to convert my spritefonts to font textures. More info on Shawn Hargreaves Blog: http://blogs.msdn.com/b/shawnhar/archive/2007/04/26/bitmap-fonts-in-xna.aspx