video displayed/actual length problem

Yeah, so I have a problem with my video codecs, which is that whenever I play a certain media file, the media player says the length of the file time is like 12 min, but when I watch it, the media player time goes past zero, as in the time has stopped counting but the video keeps playing. It’s like the media player think the viedo is shorter than it is, but the video keeps playing. And this is not a problem with my players so much, cause I’ve used media player classic, vlc, wmp, winamp, real...and I have a codec pack. Unfortunately, when I use internet clips to try to make a video with a video editing program, the problem is magnified because the part of the video that went past zero is chopped off, which makes me sad =( If anyone has a clue what is wrong, plz reply. =] thanks.