DVR-MS (Digital Video Recording- MicroSoft) is a legacy file type used by Windows in older versions of Windows Media Centre. The same file types are now called .wtv files (Windows Recorded TV). Any PC using Windows Vista and beyond will use .wtv, but essentially it is the same file type.
In terms of converting the file type, it depends what type of file you want it to be. You can convert to .wmv file using Windows Live Movie Maker, which may be included with your operating system, or else you can download it here.
If you want to convert to a different file type like .avi or .mp4, you will need to download some third party software. MC-TV Convertershould do the trick, and should remove the DRM too.