List of de facto standard keyboard shortcuts for Windows apps?
Guidelines for Keyboard User Interface Design. It contains recommendations for creating keyboard interfaces and contains, towards the bottom of the article, a list of standard shortcut keys that applications should support.
For Windows 8 Store apps, there is another set of guidelines for keyboard shortcuts on MSDN:
http://msdn.microsoft.com/en-us/library/windows/apps/hh700405.aspx