Early versions of Microsoft Office used a multi-document interface. That is, if you had multiple documents open in Word, Excel, or Powerpoint you could switch between them using a tab bar within the Microsoft Office Application. Microsoft research apparently found that this confused new users who looked to their taskbar to switch between documents, so starting with Office 2003 Microsoft changed things so each document opened in a separate instance of the application and you switched between documents using the taskbar.
One of the nicer features of Windows Vista is independent volume control for every application. IndieVolume brings this feature to Windows XP (also ME, 2000, and 2003) -- and apparently has done so for several years. This program is payware, unlike most of the programs I review, but it provides a unique feature that I haven't seen in any freeware application.
If you've been using a computer for many years, you probably remember DOS and probably have an old DOS application or two that you really miss, but just does not run well under Windows XP. Your old DOS program just does not play well with Windows XP.