and where do they go when completed
how to clear completed download
I assume you want to remove your task from BitComet… First, make sure you have U/D Ratio for it 1.0+ Then choose whether you’d like to leave downloaded files on your HDD or delete them. If it’s the former, then “Remove Task Only” (from big red X on BitComet’s toolbar); if it’s the latter, then “Remove Task & all files” (beware: downloaded files will be DELETED, not just moved to Recycle Bin).
and where do they go when completed
Usually everything goes to “Default Download Path” you may find in Options → Preferences → Task. If you changed “Save Location” on opening downloaded .torrent, then look for your files there.
It’s always a good idea to read FAQs before asking questions…