On some setups a simple key combination like ctl-alt-uparrow is set up to do it. On others, you will have to go into the screen properties.
To do this, first turn your monitor upside-down. Now you can see it. Right-click on the background, select Properties, go to the Settings tab, and click Advanced. This will usually offer a set of tabs with the last one being specific to your software drivers. Open this one (in my case, ATI Catalyst Control Center) and poke around for the rotation controls.
I hope that some key combination shortcut will do it for you.