Recently, I had the chance to load Windows 2008 R2 on a relatively dated piece of equipment, a Dell Power Edge 2850. Unfortunately, Windows 2008 R2 does not natively support the PERC 4e/xx series.
It turns out that Dell doesn't make the appropriate driver very easy to find. With the help of a forum post on their site, I was able to locate a compatible driver and load install 2008 R2.
Also, ever wonder what PERC means? "PowerEdge Expandable Raid Controller," now you know.