These guidelines are for MEP 11+ and above.
To export a WMV, use the File>Export Movie>Windows Media Export command.
MEP does a good job at exporting WMVs. Here are the settings I use for 320x240 size files that are about 2.5mb per minute (get to this screen by clicking Advanced on the export screen):
Files using the above settings will stream-play within a few seconds over a 512kbps DSL Internet connection.
By all means play around to get the desired quality verses size.
You can save your WMV settings on the main Export page by clicking on the small floppy disk icon on the right end of the Presets section at the top.
For a specific guide to exporting in WMV for YouTube, see the Tips and Tricks page.
MEP 10+ uses a different profiling system.
If you want to be able to edit the WMV profiles or create your own custom profile, install the free Windows Media 9 Encoder. If you don't, you'll get an error message when you try to edit a WMV profile. That said, I get error messages, but they are irrelevant.
To change the WMV profiles in MEP 10+, see this tute. (with thanks to Mad Magix).