So, you are tired of fiddling with codecs, and figuring out what works and doesn't work ? Well, me too.. and a few years back this really was a complete and total mess, specially when trying to deal with "codec packs" which generally just installed a bunch of things together that in many case caused more harm than help on anything, and in MOST cases never cleaned up after themselves when trying to uninstall everything from the machine.
I have always only recommended installing DivX, XviD and any extra codecs you needed separately (such as VOBFilters, QuickTime Alternative and Real Alternative) exactly to avoid the whole mess altogether. However, recently it came to my attention that latest DivX and XviD codecs are also being too overzealous on keeping the aspect ratio of videos -- which for most of us running 16:9 screens on 4:3 resolutions causes the picture to look out of proportions. So I tracked down the issue and verified that indeed at some point AFTER DivX 6 (which still works ok) the codec is forcing pixel aspect ratio and preventing the zoom feature from working properly with the built-in player in RR. Similarly after some unknown version the XviD codec, it is also not displaying what it should (even if for compatibility reasons).
Thankfully, the guys working with FFDShow have been doing a great job (as most open source projects do after awhile) and have devised enough options in their software to provide a free, reliable, light and fast solution to our needs. Those guys have come a long way fixing issues which at the early stages cause a lot of trouble with RR actually, but that I gladly say I can recommend now.
So without further a due, those steps should help you with playing almost any Mpeg 1, 2 and 4 video format that's out there.. and maybe a few others.
1-Uninstall DivX and XviD or anything else you may have that handles Mpeg-4 formats. You may leave QuickTime Alternative and Real Alternative installed, as well as VOB Filters (though FFDShow may also override it) -- this of course will be up to you regarding the file types you want to play in RR.
2-Download and install the latest STABLE FFDShow codec from HERE.
DURING THE INSTALL I DO RECOMMEND checking all available Video Types and Audio Types. Also: click to open the decoder settings at the last dialog.
3-Now on the decoder configuration screen (always available on the FFDShow icon group from the start menu), there is one KEY setting you need to change so RR works as it intended to (meaning, so the zoom choices and aspect ratio control works right in the built-in player).
This is the option that you need to MAKE SURE IT IS NOT CHECKED :
Other options you may want to set:
-Under: "Resize & Aspect", select: "No aspect ratio correction at the bottom"
-Under: "DirectShow Control", if you have "use ffdshow only in" checked, you should click "edit" and add RoadRunner.exe to the list of applications to use it with -- or you may be asked to use ffdshow with RR when trying to play videos in which case you can select "always use ffdshow" and it will never ask again.
Other options such as subtitle and postprocessing may be changed at your taste without interfering with normal functionality of RR.
From my tests here, FFDShow is actually loading and performing better than I'm used to, and offers a wide range of options, even more so than MPC provides on itself.
If you have specific MPG/MPEG (Mpeg-1 files) that are not playing, you may have a corrupt/broken file in which case you may try to fix it following THIS post.
Hopes this sorts out most video issues you may be having,