Because of many posts regarding the playability of a video file vs being able to edit that same file in Premiere, John T. Smith has written a useful article: http://www.pacifier.com/~jtsmith/PLAYEDIT.TXT
It answers many of the questions, regarding why a video file plays, but will not edit, or edit smoothly, and should be required reading, when one encounters such issues.
For some additional information, John T. has provided many useful links.
Hope that this helps address why/how a file can play on a computer, but will not edit well.
Europe, Middle East and Africa