Install Windows on your Mac, either under Boot Camp or a virtualized environment (VirtualBox, Parallels, etc.). Then you can run any Windows development environment, and any Windows programs.
If you mean "cross-develop for Windows", it's almost certainly going to be more difficult. Maybe try something like Mono for a C# dev environment. Or there's Eclipse or NetBeans, which are cross-platform. But you'd still want to do test runs on real Windows, so I don't know how well something like that would work in real life, especially if Real Life is paying you Real Money for your work.