With sharepoint you can provide custom file types to protect and I'm sure you can with WSS 3.0
On the clients side with Office you have IRM which works with RMS. There's a client which needs to be installed too.
However, this doesn't offer complete protection such as preventing a user from physically writing down the contents of a word document or using a 3rd party screen capture program. It will prevent the standard windows print screen and office apps such as OneNote etc... For this you will have to deploy the infrastructure and allow access in appropriate way, for example only allow access when on LAN and no access remotely.