I also have Windows 7 and can recommend the following 2 tools in order to play 99% of video/audio types...
1) ffdshow codec pack - http://www.videohelp.com/tools/ffdshow
ffdshow is an all-in-one codec pack for video and audio. I have used this tool for many years on many platforms (works great in my Windows 7).
2) MediaInfo - http://mediainfo.sourceforge.net/en
MediaInfo shows you all you need to know about any video or audio file, including which codecs you may be missing for that file (once installed, you just right-click the video/audio file to get details).
If you need ANY other codec type, look it up at VideoHelp.com - http://www.videohelp.com/tools/sections/codecs