There are a LOT of video codecs when you think about it. With DIVx and MPx and so forth. With each manu having their own format as well.
As for DVD, you can't watch a DVD without decoder software. Such as PowerDVD or WinDVD.
But you could try download the Klite Codec MegaPack from
http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm. As you can see by the listing, it includes a LOT of codecs! Install it at your own risk (in case a codec conficts with one you already have etc...).
Once that is installed you may want to restart. Then try playing your downloaded videos.
Lastly, you won't be able to watch some videos unless you have a license to. Kinda like WMA audio files can have a license. But that is at the whim of the video creator.
Maybe that helps.
Lastly, if you DO have a DVD decoder like PowerDVD, make sure your region code is set properly for the disk you're viewing. Next change your resolution. If you are on 1024x768 then change to 800x600 and try to watch etc... This has worked for me sometimes. Especially if your DVD has audio but NOT video. If it has nothing at all, well, there you go.
Try those things and see what happens.
Good luck