There's a "Transcoded File Cache" directory on my MCE system with 15GB in it.  Is this needed?  How much can I delete?  This is a 10% of my disk space, and I'd like to reclaim as much of it as possible.

Thanks.

I got the full path of the directory.  It's "C:\Documents and Settings\Public\Local Settings\Application Data\Microsoft\Media Player\Transcoded Files Cache\".  What gets stored in this directory?  Is there anything I can do to clean it up?

You can delete the files. Generally those files are ones that Windows Media Player creates when sycning video/audio content to portable devices. Rename them to WMV (for Video) or WMA (for Audio) and Media Center should still play them. You can delete what you wish from there.