There are a number of systems that have Windows components, or may be ALL Windows applications. However, you'll find that in some areas of the dealership, clicking with a mouse doesn't necessarily make as much sense as using a keyboard for entry. Accounting and the service departments are two areas in particular.
One system I'm familiar with is found at
www.dds-live.com. DDS has a mature core system that runs on affordable hardware. Most of the aplications are Windows-based and interact with a rock-solid UNIX-hosted central database system. If you've been using ADP, you probably wont find you're giving up anything to move to DDS, and you'll gain some. They also do not have long term contracts.
There are others, but this is the one I'm personally most familiar with.