For me, replacing the monitor driver with "Digital Flat Panel (1280x1024)" has worked since the days of Windows 95 OSR 2.1. Then, set the resolution to that level in display properties. Since LCD resolutions are fixed, you will end up with a desktop that is larger than what the monitor can display and you will be able to scroll around.
I am writing this post on laptop with an LCD panel whose native resolution is 1280x800 with monitor driver set to "Digital Flat Panel (1280x1024)" and my desktop size is 1280x1024.
Update: On Windows XP, go to the "Settings" tab in "Display Properties". Click "Advanced" and select the "Monitor" tab. I am assuming your monitor is listed as "Plug and Play Monitor". We will change it to "Digital Flat Panel (1280x1024)".
First, click "Properties" in the "Monitor" tab. Then, select the "Driver" tab and click "Update Driver". Select the option not to check Windows update. Then, choose "Install from Specific Location" and select "Don't search. I will choose the driver to install".
On the following screen, deselect "Show Compatible Hardware". Under the "Manufacturer" column, click "(Standard monitor types)". Under the "Model" column, select one of the "Digital Flat Panel" types that is suitable for you. Click "Next".
Now, if you go back to "Display Properties", you will be able to select a resolution that is greater than the resolution of your laptop's LCD panel. The desktop will not fit the screen, but you will be able to scroll around.
Of course, I cannot guarantee that this will work with any given external projector but I personally have not had any problems using this technique with a variety of devices in various classrooms over the years. I also cannot say anything about Windows Vista or Windows 7 because I have not used them at all.
You might first want to try this without connecting to any external displays. Note that this technique will not work with CRTs.