Even though Windows XP is beginning to show its age in some areas, it sill has support for most media file formats, including MP4. Any media player installed on a Windows system should support MP4 playback, but if for some reason the file is not associated with a program, then you might have a hard time getting it to play. If that is the case, you can associate the file to Windows Media Player, which comes with almost all versions of Windows, to play the file.
Right-click on the MP4 file you want to play and select "Open With." In the dialog box that appears select "Select the program from a list" and click "OK."
Choose "Windows Media Player" from the list of programs that appears and select "Always use the selected program to open this kind of file." and click "OK." If Windows Media Player does not show up on this list proceed to step 3.
Click "Browse" and go to the directory where the Windows Media Player executable file is located. This file can usually be found by going to the "C" drive, then "Program Files," then "Windows Media Player." The file's name is usually "wmplayer.exe." Select that file as the program you want associate with MP4 files and click "OK."