The guidelines for Windows 8 desktop applications are the same as for Windows 7.
That means that you'll find them here:
Windows 8 has two separate UIs: Windows Store apps (formerly known as "Metro-style apps") and Desktop.
Windows Store apps is the new, future oriented touch UI. These apps will typically run in touch environment where consumption (not production) of content is the main purpose of the app.
The Desktop UI is there for backward compatibility(!). It will be used by applications that need the full power of Windows. This UI is similar to Windows 7. Only minor changes to the look and feel, eg. squared button corners instead of rounded.
- http://www.windows8designhandbook.com/ (Suggested by @Simon)