Check in your display properties (it's going to be interesting with a
sideways desktop) or in the system tray near the clock for a display
"utility," many will have a desktop rotation option. Can't be more specific,
since they're all a bit different, but they're usually pretty
straightforward. In display properties, it's often a tab with "ATI" or
"NVidia" (or whatever other chipset you may use, like intel, matrox, etc.)
on it.