Without any additional codecs installed, Windows and OS X don't share a lot in common.
In the order of popularity and age:
- MPEG-4 Part 10 / H.264? Pass. It's too new (2003) and would need a codec pack on older Windows versions, and it has some licensing issues.
- MPEG-4 Part 2? Won't work as well. This is the codec that needs DivX or XviD installed.
Both of the above should work in Windows 7, but there's of course Vista and XP as well. Now we're talking old:
- MPEG-2 is still problematic, although it's from 1995. Why? Not every Windows Media Player can just play it.
- MPEG-1 could work. It was developed in 1988.
Yes, I'm not joking. This is the least common denominator. You can find a list of supported video codecs for Windows Media Player under Windows XP from Microsoft. It basically says: "Only Microsoft-specific stuff". Here's what they have to say:
There are hundreds of audio and video codecs in use today. Some have been created by Microsoft, but the vast majority of codecs have been created by other companies, organizations, or individuals. By default, the Windows operating system and the Player include a number of the most popular codecs, such as Windows Media Audio, Windows Media Video, and MP3.
Emphasis mine. I won't add any further comment to that, but you can guess that it's just wrong to say that Windows Media Video is among the "most popular codecs".
On the Mac, it's not a big issue to decode most of the above mentioned, I'm pretty sure that only Windows Media Video can't be played back on a recent Mac (running 10.5 and above) without additional codecs.