|Do the movies play when you launch them directly from your PC?|
If they also don't play, It would seem you're PC is a bit too slow & you have said your computer goes slow. If that's the case you may want to upgrade your PC so it will play movies better.
If movies play from your hard drive and only fail on the pendrive, It's possible the file is corrupt, the pendrive is faulty or corrupted.
Try putting it into another USB port.
You could also try formatting the pendrive (My Computer > Right click pendrive > Format), which will erase everything stored on it. Then you would put the movies back on from your PC.
Or you could try a new pendrive if that doesn't work.
PEBMAC - The biggest cause of Computing problems.
Damn ID10T Errors.