Google Cloud Connect is a free plugin that improves Microsoft Office 2003, 2007 and 2010 on Windows PCs. It adds simultaneous collaboration, revision history, cloud sync, unique URLs and simple sharing to the Microsoft Word, Excel and Powerpoint applications.
However, according to a blog post by Google Operating System (unofficial Google blog),
I created a new document in Word 2010, but Google saved it as a read-only Word file in Google Docs. Apparently, the document can only be edited using Microsoft Office and not using Google's online word processor. Since you can't even open existing files from Google Docs, this software seems to be too limited. It's useful if you and all your collaborators only use Microsoft Office and Google's plugin.
Download Google Cloud Connect for Microsoft Office from http://tools.google.com/dlpage/cloudconnect