If you are looking for a secure document management system, SharePoint can do a lot for you. Still it remains a web based application and when a document is downloaded it can have its own live. Users can do local with the document as they please.
Not anymore, windows came up with: Rights Management. Making it possible to extend your security to where many thought it was not possible. You can secure a document to: read only, but also not making local copies, printable or not etc. I included some screens to show the possibilities of RMS.
This little icon will be visible in your office application:
When you click this icon, this screen will appear:
You can set and change rights. Click on More Options to get more secure:
Together with WSS 3.0 I feel I have a secure and stable environment.