I use this one. Shark007's FREE Codec solutions - Windows 7 codecs Then just set it to suggested setting's. However I do set it to use FFDshow DXVA codec for H264.
+1 for the Shark 007 codec pack, I have used that since the Windows 7 betas. You really do just install it and Windows Media Player / Center will play anything. If you enable DXVA (requires a supported graphics card) you can play HD content with little CPU usage, so even the a low powered Intel Atom PC can play HD content.