As this is being posted in the ExtendIt! (Programming with VS 2005, ASP.NET 2.0) forum, I'm assuming that you are working with the DNN 4.00.02 or .03 source.  Because of the changes in the folder and compilation model of  DNN 4 and VS 2005, only the install PA's of the core module are included in the DNN 4 source.  You can download the source version of the core modules by clicking on the core module project name at the bottom of the Downloads page.  For the Announcements module, the download page will be: http://www.dotnetnuke.com/Default.aspx?tabid=926.

Note however, that these sources are still those of DNN 3 and have not been converted to the new folder/compilation model so cannot be built without first making changes to the folder structure and possibly some namespace changes.  Recently I did convert a third party module source by hand - not too time consuming, but due to a lack of proper namespacing in the original source, had to make many changes in that area. Although I have not yet tried it, you may be able to use the new release candidate of the Web Application Projects addin for VS 2005 to enable you to work with these sources using the old VS 2003 single assembly compilation model.

At some point, I'm sure the DNN 3 modules sources will be converted to DNN 4/ASP.NET 2.0/VS 2005 format.