When you refer to "Windows developer preview", exactly what software did you install? Are you using the Windows 8 DP or to Visual Studio 11 DP?
In either case, you have installed pre-beta software and compatibility issues with existing production software are to be expected. If you had no problems with XNA before installing the pre-beta software, I suggest you remove it to restore the functionality
you want.
If you have questions on the Developer Preview software, here are the forums you want:
http://social.msdn.microsoft.com/Forums/en-US/category/windowsapps