Problems you're having may include, but may not be limited to the following:
1: The streaming video will not play: The solution here is to get the latest Flash player.
2: The downloaded video won't play. Here you have two options. First, get MPlayer(you really should get it regardless, on principle). Your second option is to download and install XviD.
That should do it. Unless the video is too dark. If that's the case, you may want to consider purchasing a Geforce Ti4200, not forgetting to get an OCed PSU from BFG, of course. If neither of these solve your problem, then it could be something on my end, so I'd appreciate it if you sent me an email at PerXes@gmail.com