== Reason for DOS incompatability == 

 "[Windows XP] still has trouble running many old DOS programs. This is largely due to the fact that it is a Windows NT system and does not use DOS as a base OS." 

 DOS not being used as the base OS has little to do with it. The main difference between XP's virtual DOS machine and a real DOS machine is that the VDM doesn't allow access to the real hardware. Couldn't software running as part of an OS that does "use DOS as a base OS" disable direct hardware access just as easily as NT does? -   |