DosPad, which was previously known as iDos, is a universal app that supports Keyboard, multiple screen modes and allows you to install Windows 3.0 on your any idevice i.e., iPhone, iPod Touch and iPad.
iDos is based on an open source DOS emulator and the main purpose of this app is to install the DOS based games on your idevice. However, it does more than that. You can also install Windows 3.0 on your phone with the helpf of this app.
I don’t know whether it comply with all the guidelines of Apple or not, but it was approved and soon pulled back. Now iDos is available in Cydia store for all jailbreaked device with the name of DOSPAD.
To jailbreak your iPhone, you can either use Greenpoison or Limera1n if you are on supported carrier. And if you rely on unlock and have got an idevice running on latest iOS 4.1, you must use PwnageTool (for Mac users), or Redsnow ( for iPhone 3G and iPod Touch 2G, both Mac and Windows ).
For more details, head over to this active thread on TouchArcade Forums.