Sound no image. Perhaps video codec related. Did you add or remove any dvd software or media player of any kind recently? Two years ago I had a PC that would not play WMV after I installed RealPlayer. I couldnt completely get rid of it, and the WMV issue persisted. Only the drastic step of a reinstall of Windows restored it. Players often work very hard to assert themselves as a default player for a whole host of media types and make a host of registry changes in the process.
When you say trouble downloading... Is it not able to have the file fully transferred and saved to your hard drive first, or, unable to play a downloaded video after its completely transferred and saved on your PC?
Attempting to play a file before its fully downloaded wont always work. Not sure if this is your experience with download: viewing a video as its being sent to you is given the term progressive download, and will be very sensitive to your network conditions. If your internet is WIFI, the environmental senstivity can be very high. On PC its left click vs the right click. Attempting to (watch) a file from the download section, as it is being downloaded, (left click) is not recommended. You dont enjoy all of the benefits of streaming. The two methods (download vs stream) work very differently. Just asking so this can be ruled out as a possibility.
I think you can rule out bandwidth if you can watch the stream and the issue is isolated to downloads. Streams by the way are delivered as MP4s and rely on different codec.
Can you play the MP4s?
Hope this helps