Hi, since your wmp plays the file I don't think that this is a codec issue here. I'm not trying to be mean here but it sounds like your computer just doesn't have the processing power or enough memory to run the video smoothly as it's probably an hd video. Here are a few things you may check,
1. press control alt delete and close any unnecessary processes with task manager while playing the video that may be eating up memory (ram)
2. make sure your video hardware acceleration is set to full, this article covers how to do that from win 98-vista http://pptfaq.com/FAQ00129.htm
3. maybe try a different media player such as vlc, gom player, media player classic or media player classic home cinema, kmplayer or it's new remake daum potplayer english version (kmplayer does have an UNINSTALLATION FLAW and the new vlc may not work right on some older computers so you may get an older version of vlc from http://www.oldapps.com/VLC_Player.php (I use old version vlc 1.0.3)
My favorite media player is starting to be the new kmplayer called daum potplayer but it is still in it's infancy and being developed but it does support nearly any formats and full hd video. Here is info on daum potplayer http://forum.doom9.org/showthread.php?t=148745
Here is the latest files for daum. First install daum then install the update for it, and yes it is in english. http://www.dvbsupport.net/download/index.php?act=view&id=230 install this first, then this update, http://www.dvbsupport.net/download/index.php?act=view&id=239
After installing daum I recommend you right click and change the skin setting to this, (potplayer default skin-original_new_en.dsf) then go through and change all the settings to your liking. You can manage file associations by, [right click on daum - click on preferances - click on association - to the right click on defaults - click on select all - then apply and close it out, ALWAYS select none then apply before uninstalling daum or kmplayer.]
I hope this helps you out and if you need further help add some additional details and I will do my best to help you further. Good luck!