BS.Player version and build nrs?
Did you install BS.Player with default settings?
If you change some setting, does it "stick" (after having closed and re-opened BS.Player)?
Perhaps "Register extensions on player start" is selected under
(rightclick >) Options > Preferences > General > Filetypes ?
Anyhow, I also suspect this may well be an UAC-issue.
All required (writing) rights & privileges for BS.Player are OK?