Multiple (dual) monitor issue

Hello,
I just purchased 2 Dell UltraSharp 1908FP monitors to connect to an HP Pavilion computer, graphics card 1 GB Radeon HD 4850 (2 DVI, HDMI, and VGA adapters). When I right click on the desktop, go to Properties, Settings, Advanced, Monitor, I am not given any choice of monitor or the dual computer graphic I see on other computers
here in my office.
It shows "Default Monitor" and "properties in this window is grayed out.

I wanted to expand my workspace to both computers but all I have is a clone right now...

My operating system is Windows XP professional x64.

Many thanks,
Turkey-lurkey