Hi, I really do not recommend the k-lite codec pack, eventually it can break your system because not everything it installs is compatible with each other. I do have some recommendations though,
http://www.gomlab.com/eng/GMP_Introduction.html This is the Gom Player. It is free and very simple to use and has most codecs built in to play video files. If it doesn't have the codec you need it will take you to a site to get the proper codec.
http://shark007.net/index.html This is the best codec pack i've seen been using it a long time without any problems and it will not break your system like k-lite will. Go there and download the pack for whichever version of windows you may have, the vista version is compatible with xp. You have to go to the bottom of the page for the download link which will be majorgeeks. Do not believe the crap about this pack containing a virus thats a false positive and the owner of the shark site has done submitted the problem to all the major antivirus companies. You may want to check out his version of the media player classic which is updated as well under the shark007 tools section. This pack can also make windows media player the default player for all video files if you choose that option in the shark007 options window.
http://download.pandora.tv/?mode=kmp&tab=1 This is my personal favorite media player 'the kmplayer' which is not to be confused with kmplayer the front for mplayer. This player is completely independent from mplayer and is very customisable and skinnable. If you take time and go through all the settings and change them to your liking it will be the only player you ever really need. It has built in codecs and can also handle external codecs and filters. It can do everything ffdshow can.
Also the vlc player as others have suggested is very good but video quality isn't the best at times. Kmplayer has very good quality. Whichever you choose I wish you the best of luck. Hope this helps.