Depends on what kind of complexity you are after. Lightroom would the direct equivalent of Aperture (except of course, it's a Beta at the moment, and I've heard the Windows Beta is not as good as the Mac beta).
If you need to do some extra pixel-editing, then the suggestion is to go for Adobe Photoshop Elements 4. As well as a cut-down (but still pretty powerful) editor tool, the Organiser component is pretty usuable. In fact, I quite like Elements 4 on Windows... before I switch to Mac.
Elements 4 can handle RAWs, but you can't batch process them.