Dell Power Edge 840 memory problem

[Little Marine]

I bought a Dell Power Edge 840 from a friend of mine to use in my server class. It has 2 gig of ram but the computer properties says only 256 megabytes of ram is available. It has Windows XP Pro on which I want to get rid of. I want to put Windows Server 2008 on it but it won't let me because there is not enough ram available. I ran a memory test with no errors. Is there anyway to get my computer to see all of the available ram. It has a Xeon processor.

[Transfusion]

Occam's Razor states that the simplest solution is usually the best. I was about to think of stress-testing your CPU, moving your RAM around in different combinations, running in single channel and dual channel mode....

http://supportapj.dell.com/support/edocs/systems/PE840/en/hom/pdf/hom.pdf 

OS Install Mode
(Off default)
Determines the maximum amount of memory available to the operating system.
On sets the maximum memory to 256 MB. Off makes all of the system memory
available to the operating system. Some operating systems cannot install with more
than 2 GB of system memory. Enable this option (On) during operating system
installation and disable (Off) after installation.

Houston, We have a solution.