In the Checkout Dialog, there is a button named Fetch list.... Clicking this causes TortoiseCVS to attempt determining which modules exist on the selected CVS server. It does this by using the following algorithm:
Run the command cvs co -c to obtain the contents of
CVSROOT/modules. Note that the CVS administrator must
maintain this file manually, so it might be out of date (or empty).
Run the command cvs ls to obtain all existing modules. This is a reliable way of obtaining the module list, but only works if the server runs CVSNT or a recent version of cvshome.org CVS.
Attempt to guess the Web Log URL (see How Web Log Autodetects the Server URL) and determine the available modules by parsing the output from the Web Log server. This of course only works if the CVS server provides Web Log.