I eventually found the answer: In Premiere Elements (with a project open; it can't be done without a project being open), select Edit > Preferences > Media
This will show the location of the Media Cache Database and provides a button to clear that cache. Clearing that cache will apparently only clear it of files that have 'lost' their sources, so if you use a USB disk for your projects, the you'll need to remove the disk before clearing the cache. To do this you'll need to exit Premiere because you've presumably got the USB open ..... Arrrrhg!
I found this to be useful:http://provideocoalition.com/ssimmons/story/Clean_those_adobe_media_cache_files
What version of Premiere Elements are you using and what is your Windows Vista 32 or 64 bit?
If you are looking to regain some more free hard drive space, please consider.
1. Removal of the preview files. If Adobe default, then Documents\Adobe\Premiere Elements\version number of Premiere Elements\Adobe Premiere Elements Preview Files Folder.
2. Removal of conformed audio files (cfa and pek sets), then Documents\Adobe\Premiere Elements\version number of Premiere Elements\Media Cache Folder.
3. Removal of conformed video files (.mcdb), then Local Disk C\Users\Owner\AppData\Roaming\Adobe\Common\Media Cache
Running the ccleaner (regular and registry cleaner) for overall computer optimization (be sure to say Yes to backing up registry before the registry cleaning...never had any problems, but good to be safe).
Good idea to check Edit Menu/Preferences/Scratch Disks for the reading of available space at the location to which they are directed.
General thought...check out Adobe Folder Media Cache for cfa and pek and that Common Folder via the Users directory for .mcdb unless you have custom locations. Check periodically to make sure that the Scrath Disk Locations have not changed without you knowing it.
Also, go easy on System Restore since the process eats up a lot of hard drive space.