The ability to do this is hidden away, not in the preferences and may not be immediately obvious.
To do this you need to change certain settings in the Site Manager, unfortunately you cannot set these options globally, so you will need to do this for each of your ‘Sites’. In Dreamweaver CS5 your Site Manager should look like this:
Dreamweaver Site Setup
Once you have the Site Manager open, expand the Advanced Settings tab and you should see the ‘File View Columns’ option. From here you can select which columns you wish to show, simple double click on them to bring up the column options window, like so:
Dreamweaver Site Setup Options
Check the box ‘Show’ to Hide/Hide a specific column. That should be it, you can now hide/show specific columns that you wish to see in the Files window of Adobe Dreamweaver CS4 or CS5.
Remember to do this for each Site.