Creating Word document icons is very simple thanks to a service called LiveDocx. LiveDocx was created as a web service to allow the easy creation of most document formats from a simple template. However, it is possible to send a normal Word document as the template file and get an image of the file in return.
You'll need a LiveDocx account to be able to use the service - there's a free option of their service that uses a shared server. Included in the post is a sample script that defines a LiveDocx connection, pulls in a local Word document for parsing and calls a "getBitmaps" method on the service to return the raw image data. This is pushed into an image (using GD) as a PNG.