for codecs problems you could download a pack ... if you are not an expert in codecs it helps to get them from people whose knowledge is at least above your own ... (and it's a hassle to watch for new codecs every time).
personnaly I think 2 packages are good enough the first one is CCCP : good and reliable for all palyback I found it was the more stable and accurate ... it comes from the anime subbing cmunity and most groups see it as a standard ... many other encoders adopted it too for their release.
this package is on playback oriented ... if you want to encode yourself then k-lite might be a good idea less stable but with more options (although options are good to tweak when you know what you do total noobs shoild know better and stick with CCCP)
CCCP can be found at
http://www.kickassanime.tk/ (great community support there too).
K-lite can be found at
http://www.codecguide.com/ (my recommendation don't use their "CCCP settings" it's just here to attract CCCP user and I found it unstable. )
both packs have the MPC (Media player Classic) included this player is one of the lightest and less bugged player on windows it is fully compatible with every codec a must have (looks like MP 64 though but who cares in full screen mode ...)
another Media player with good GUI and that works with those packs is The Core Media Player ... nice looking friendly and efficient . available at
http://www.tcmp.org/
Remember once you chose your codec you only need to watch for a new version of the same one when you can't play a file ... uninstall the old reinstall new one and it should work without a problem ... (if you have though, forums and faqs should address the problem better for a pack than just an element because they use the same things as you do).
my opinion from personnal experience (got 5 PCs on ZinXP those solutions worked with every single one of them):wink: