Hi, I have a solution to your codec problems, why install other media players when you can make windows media player work for most files. Try this, this is what I use and trust.
http://shark007.net/index.html
The above link is the shark007 codec pack. It will make windows media player play all files by default if you go into the shark007 options settings window under the help tab. Some files that it will make windows media player default for are flv, mkv, avi, rmvb, almost any media format. It will enable you to playd dvd's.
Just go to the shark007 website and choose the shark007 pack for whichever version of windows you have, the vista pack is compatible with xp. You have to go to the bottom of the page for the download link which will take you to majorgeeks website for the download. This pack does not contain a virus, it is a false positive in which the owner shark007 has already submitted to the major antivirus companies for review.
Heres some tips. If after you install it and windows media player will still not play a dvd, go into the shark007 settings option window. Click on the first tab that says config, then click on ffdshow video config, then under the codecs tab find the mpeg2 which will probably say disabled, either choose libavcodec or libmpeg2 and click on decode dvd's at the bottom and click apply then ok. It should then play dvd's. You may want to also go into the ffdshow audio config and go down to processing and uncheck everything excpept (16 bit Integer). You can also turn off those annoying ffdshow and haali media splitter icons in the shark applications settings window under misk tab.
You may want to check out shark007's udated version of media player classic or mpc home cinema under the shark007 tools section of the website close to the bottom of the page. They play dvd's well.
One more thing, if you go to uninstall the shark007 codec pack use the uninstall option in the shark007 settings application window under help and not the regular computer uninstaller program. Please do not use the k-lite codec pack, it can severly break your system because some of the stuff it installs isn't compatible, i've found that out 3 times now. Shark007 has never hurt my system.
http://shark007.net/forum/ Go here for any further help you may need with this codec pack, shark007 is an expert on this stuff.
IIf you still can't play dvd's after trying all this, you probably don't have a dvd rom on your computer, you probably have a regular cd rom or cd-rw which won't play dvd's.
If you do want to try the vlc media player as other people suggested, download it from the official site. it does not use external codecs one bit and plays nearly any file,
http://www.videolan.org/vlc/ Official site of videolan vlc player.
If this all sounds familar its because I just answered a question the same as yours.
I wrote this for another question on here but the same would apply to your question. You can try vlc media player also. Gom player is really good to for avi's and works with the shark007 pack to even play more files. I would recommend the kmplayer but its not for beginners its got too many options so try the shark007 codec pack, vlc, gom or media player classic.
http://www.gomlab.com/eng/GMP_Introduction.html Gom Player.
I hope this helps you out.