If your video card supports rotation (most newer ones do), you can get the latest drivers from the manufacturer's website, which should include a utility that lets you set the rotation. This was the case for the nVidia card I had at my old job, anyway.
If your card doesn't natively support rotation, there's a product called Pivot Pro that lets you do it:
http://personalcomputing.portrait.com/u ... rview.html
I liked it, but I'm too cheap to pay for it.
Rotation is nice because you don't have to scroll as much. After using it, you start to realize how silly it is that computers come with horizontal displays, even though most of the things we look at are vertical in nature (web pages, Word documents, PDFs, etc.).