Pegasus has a "System mailer" button [Tools:Options:Incoming Mail:Hyperlinks] which is supposed to set Pegasus as the default email client in Windows. It works fine in Win2K and probably XP, but not in Vista.
The problem is that Vista has a new Registry entry that Pegasus does not know about -- HKLM\SOFTWARE\Classes\mailto\FriendlyTypeName
which points to Windows Mail. This registry entry must be deleted. The System mailer button takes care of the other two registry entries OK (DefaultIcon and shell|open|command).
John (loyal Pmail user for so long I've lost count -- ~19 years)