This is going to be a large and very involved project. I've been responsible for migrating Access databases to .NET many times, but never on this scale. I agree that a gradual approach will probably be the most realistic. Trying to take on everything in one shot is an easy way to fail.
As in other answers, the first and most important step will be to move everything to SQL Server. While doing this, document the database if this hasn't already been done, and identify areas for refactoring if they exist. Access databases that grow to meet evolving needs often have data models that can be improved upon when looking at the big picture.
Next, identify modules of the application that are "self contained". Because this is a do-everything database, there will probably be modules of it that are nearly completely disjoint from each other. After having these disjoint pieces, identify one of the smaller modules that has the most to gain from being updated, and take it on first.
While migrating to .NET, don't just do a simple 1 to 1 copy of the application. Gather input from users to identify pain points with the current application. You want your first migration unit to be a huge success to gain the complete buy in from everyone else.
After you have the first unit done, look at what happened in terms of challenges, difficulties, etc, and use this moving forward.
One thing that I would strongly discourage would be to implement partially working modules (ex: "we migrated the CRM, but features X,Y,Z are not in the new system yet"). This will cause users to quickly become frustrated from having an incomplete system when the old one to them was "perfectly fine". This kind of development may work fine for other domains, but not in businesses where users see "nothing wrong" with the old Access monolith and don't understand the migration needs.
Best Answer
Although it is certainly possible for a Windows service to listen for http requests (that's what IIS does after all), you cannot have a Windows service listen for commands to start and stop itself. That would be a paradox. How can your service listen for a start command before it is started?
The key is to have a second service control the first one. Write a small web service to accept the start/stop/restart commands, host it in IIS, and pass the commands down to the Windows service control manager. You can use any convenient technology for this, ASP.NET Web API or WCF will do fine.