Just to make this a little easier, you should have a peek at the recently released Group Policy Preferences, which allows you to set a plethora of settings, such as file associations, straight out of the GUI. It will need the updated Client Side Extensions to be installed....but it is truly worth it, as GPP makes it a breeze to configure things like this.
* Applications: Extensible for ISVs to create simple Group Policy preference settings for their applications.
* Drive Maps: Create, modify, or delete mapped drives, and configure the visibility of all drives.
* Environment: Create, modify, or delete environment variables.
* Files: Copy, modify the attributes of, replace, or delete files.
* Folders: Create, modify, or delete folders.
* Ini Files: Add, replace, or delete sections or properties in configuration settings (.ini) or setup information (.inf) files.
* Network Shares: Create, modify, or delete ("un-share") shares.
* Registry: Copy registry settings and apply them to other computers. Create, replace, or delete registry settings.
* Shortcuts: Create, modify, or delete shortcuts.
Control Panel settings
* Data Sources: Create, modify, or delete Open Database Connectivity (ODBC) data source names.
* Devices: Enable or disable hardware devices or classes of devices.
* Folder Options: Configure folder options; create, modify, or delete Open With associations for file name extensions; create, modify, or delete file name extensions associated with a type of files.
* Internet Settings: Modify user-configurable Internet settings.
* Local Users and Groups: Create, modify, or delete local users and groups.
* Network Options: Create, modify, or delete virtual private networking (VPN) or dial-up networking (DUN) connections.
* Power Options: Modify power options and create, modify, or delete power schemes.
* Printers: Create, modify, or delete TCP/IP, shared, and local printer connections.
* Regional Options: Modify regional options.
* Scheduled Tasks: Create, modify, or delete scheduled or immediate tasks.
* Services: Modify services.
* Start Menu: Modify Start menu options.
* Group Policy Preferences FAQ
* An Overview of Group Policy Preferences