The command line that Windows sends to the BitTorrent program is stored in the registry, with the file-type association for TORRENT files. It can be changed as follows:

1. Go to Folder Options in the Control Panel.
2. Select the File Types tab at the top of the dialog.
3. Wait a moment for Windows to load the list, then scroll down and select the TORRENT extension. Tip: press the 't' key to jump to entries starting with t. Note: The file extension column might be truncated to just TORR...
4. Click the Advanced button.
5. In the list of Actions, select open, probably at the top of the list, then click the Edit button.
6. The contents of Application used to perform action is the command line. By default it should be:
   
   "C:\Program Files\BitTorrent\btdownloadgui.exe" --responsefile "%1"
   
   Note the double-quotes around %1, and your Program Files directory might be different. To add more command line parameters, simply change this string. For example, to change the port range, you might use:
   
   "C:\Program Files\BitTorrent\btdownloadgui.exe" --minport 10000 --maxport 10100 --responsefile "%1"
   
   See What are the command line parameters for the BitTorrent client? for a list of all the available command line arguments and their defaults.
7. Finish by clicking OK to close all the dialogs.

Also note that if you are using TorrentSpy to launch BitTorrent, the command line used is contained in the TorrentSpy configuration. Use the System tab in TorrentSpy to modify the command line parameters used in this case.