Hmmm, it boils down to this, as for XCode, you can't install it on Windows.
But... Depending on the app you are planning to develop, there are some environments in Windows to do so, for instance, games, you have UDK, Shiva, but eventually you'll need access to a Mac computer to pack things up, but it can be virtualized (hackintosh of sorts).
Or you can opt for a cloud-based development environment like Moai SDK.
Also, there are some SDK/Toolchain independent solutions (no Mac required) - Marmalade, Adobe Flash->Air.
Please note that I'm mostly into gamedev, so, all of these probably won't fit good if you're digging into app development.