There are tools that allow to run fullscreen apps in a window. They're doing this by hooking video initiation functions and changing flags so that the initiaton prepares program to run in a widow. Additionally, those programs must hook and modify other functions, because pograms which are assumed to run fullscreen usually use mouse support and video modes in a way which assumes there's nothing else on screen.
Which tool is the best one? It's hard to tell; it depends on your personal preference and what you wish to achieve. D3D Windower is definitely the most featured one, while DXWnd is very light and doesn't eat up much of your system resources. There are also other such tools; I'm discussisng only these two because they are most widely used.
Note that these tools are hooking themselves to another program and modifying it. Such actions are similar to what some viruses do, so heuristic anit-virus methods may recognize some of them as suspicious. These files are NOT infected with any viruses; you don't have to believe me, but please don't argue with me in that matter. Still, I'm not taking any responsibility for this code; use it at your own risk.
A very simple tool, but still it will work for most games. The program is written using Visual C++; it is fast and doesn't require many resources, but there are only few configuration options. Will only work for DirectX applications.
This tool was published by his Japanese author with source code, and became the root of all other such tools.
English Win32 version 1.21a, download link: hxxp://keeper.lubie.org/tools/sfb7s_dxwnd_v1_21a_en_win32.zip
Download japanese source version 1.20
If you trust your AntiVirus more than me (maybe you should), then don't download the w32 version.
A clone of DXWnd which is worth mentioning. Provides more options than original, is completely english and easy to use. Also, keeps the simple design and is relatively light.
Definitely the most advanced DXWnd clone. It is written using C++ Builder, which makes it quite heavy; but it has a lot of options, and supports more applications than other tools. It can hook a lot of OS functions, and may work even with non-DirectX apps.
Originally written in Japanese language, it was translated to english with use of reverse-engineering techniques. Not all of it is translated, so sometimes you may see a message box which is quite hard to read.
This program represents completely new line of windowers, and also has other functions. Works only with 3D apps; I didn't tried to use it with DK, but it should definitely work with DK2.
The 3D Analyzer Tool was created and refined to overcome limitations posed by modern 3D Games and other Applications on several current mainstream 3D Cards.
Please report any problems or dead links.
Read README file before using any of the tools!