I have been meaning to report this for a while, but I installed Azureus today, and it registered itself properly, so there’s no reason why BC can’t.
When I install BitComet under Win7/64 Enterprise SP1, it does not set up the necessary registry entries to identify itself as an installed program. I must go into the registry and insert these keys by hand into HKLM\Software\Classes\Applications.
(I do not know if this also affects Win7/32 or other versions of Win7/64.)
Without these entries, Win7 will not accept an association between BitComet and .torrent files, so will not become the default bittorrent client.