If your currently using Windows 7 you should notice that the Windows Media Center (WMC) is better integrated with cable, broadcast and internet TV. Windows 7 now no longer uses DVR-MS format. So if your going to play these files on older version of WMC or Windows Media Player, your out of luck. The only way you can watch them on older versions is to convert them back to .dvr-ms.
With Windows 7 you can easily do this by just right clicking and converting back to .dvr-ms.