If VLC won't play them then it sounds like those files might have a problem. Do they have an extension? eg: file name.avi
Can you do a test for me to see if there''s any interference in LW's incoming:
Bug Report instructions & sample image IS this true/false?
Do you have firewalls? If so, have you allowed LW access thru them. There is a chance you're receiving damaged download packets due to a firewall or nat or something else.
Did you dwnld these files from T1 or T3 sources or cable/dsl or modem?