It sounds like you need to install a codec pack like this one
http://www.free-codecs.com/download/K_lite_codec_pack.htma collection of codecs and related tools. Codecs are required to encode and/or decode (play) audio and video. The K-Lite Codec Pack is designed as a user-friendly solution for playing all your movie files. With the K-Lite Codec Pack you should be able to play 99% of all the movies that you download from the internet.
Please try it and let us know if it resolves your problem.