Best format to convert to for WMP?

[petiteGMT]

Greetings:
Can anyone give me some suggestions on what format is the best to convert to so I can also play video on WMP and Corel's VideoStudio? Corel is pretty flexible. But WMP has been a problem, given that in Windows 7 Pro 64-bit they do not provide the codecs to play mp4. I've been asking on the microsoft forum, and there was quite a bit of angry discussion on that before I got to the topic. They put out WMP, they ought to provide the codecs.
Anyone know where one might get the appropriate codecs (from a clean site? ie: no viruses or malware).

Or, in the alternative, can you suggest what format to convert the video to so it will play in WMP? VLC is versatile but WMP is not, but I must use that for a reason.
Thanks everyone in advance,
GMT