There are a few options available to those wanting to run Android apps on a Windows machine. The first, and oldest, is the official Android emulator; there's also Bluestacks, which has been around for a while. Today another contender emerges -- WindowsAndroid, from Chinese startup SocketeQ. Unlike the emulator, it runs Android natively on your Windows PC, and unlike Bluestacks it brings to life the full Android OS, not just individual apps. Essentially, it's full-blown open-source Android running natively, in a window, on Microsoft's desktop OS.
The current version is based upon Android 4.0.3 Ice Cream Sandwich; SocketeQ says that a "new version" is being planned, presumably based on the newer Jelly Bean. It uses the Android tablet UI, and you can control things using the keyboard and mouse.
On the whole, it's a pretty speedy, if slightly buggy experience. We used WindowsAndroid on a 3GHz Core 2 Quad rig -- by no means bleeding edge -- and the whole OS ran smoothly without taxing the CPU too much. There are some glitches to watch out for, though. Certain tasks, like trying to load the camera, will result in a crash, and loading some websites in the browser will result in an error message, accompanied by missing images. So it's still early days for WindowsAndroid at present.
What's great for hackers is that the entire directory/partition structure of Android can be fiddled with directly through Windows -- just navigate to C:\SocketeQ\windowsandroid_root, assuming you've installed it to the default location. That means sideloading apps is simply a matter of loading the relevant files into /data/app.
The WindowsAndroid installer weighs in at 64MB, and takes up 300MB when fully installed. You can find download locations over at the source link; it's also mirrored elsewhere if you're having trouble downloading via the overloaded official server.