Hello There,
There are a number of reasons why this can occur. One of them is the use of
an incompatible codec....Don't worry about what it is
Go to my website and download the program "Rename Codecs" and take a look
at the FAQ > Rename Codecs and read how it works...that won't take much
longer than it does to download the program.
Use the program and if it does not solve your problem please come back and
say so. Include in your reply the CPU type and speed of your computer, how
much RAM you have and how much free space on your hard drive.