I have found that if you delete the problem files on your hard drive, then go to recycle bin and restore them, WMP will then read the track lengths. Sometimes you get a couple of tracks that haven't repaired themselves, but just repeat the process with those tracks and eventually all your tracks will have the length time attached to them in WMP.
P.s. Please make sure you have got your pc set, so your deleted files will go to recycle bin and not be deleted forever.
Sorry for a rather late reply to this post. Hope it helps someone