Can't export timeline as a Windows Media file anymore.

Question

Using Premiere 5.0.3 for over 2 yrs now.  No problems until now.  I first export my 30 min. timeline as a H.264 file as a master.  No problem.  Then I would export my timeline as a Windows Media file (NTSC Widescreen) for easier viewing on other computers.  Again, no problem - until  last week.  Each time I try to export now as a WMV file, it takes the usual over an hour to do the conversion, but then when I look at the file information after it's completed, instead of the approx. 400 Mb file size, it will be some other smaller file size anywhere from 4 Mb to 217 Mb.  Everytime I try to export again, (and waiting again over an hour each time for the conversion to complete), the file size will be something considerably less than it should be and different each time...but the time will always be what the video should be - approx. 30 min.  If I view the video, it will abrutly end corresponding to the file size, IE - if the file size is 200 Mb, the video stops at about 15 min.  What happened?  Nothing changed in my computer.  No new program was added.  No settings were changed...and my editing computer is not connected to the Internet so as to avoid any possible contaminaton from outside sources.

Answer 1

Thanks Jim for your responses.  It's suggestions like yours that get me to think in another direction, which is a good thing.
I never suspected anything wrong with the hard drive or any corresponding components because the drive worked fine otherwise - playing back clips already on it, transferring files from one drive to another, etc.  At any rate, I opened up the computer case and swapped out the SATA cable to that drive (I have lots of spares of everything) and then did a defrag and check of the drive which all showed good.  I then exported a couple more videos in the WMV format to that drive and everything worked fine, so hopefully all is good from now on.
Again, thanks for your help.