http://www.pmail.com/vista.htm

“Pegasus Mail: For reasons not adequately explained, Windows Vista does not include a functional help system (WinHelp has been removed, and HTMLHelp has been crippled). This means that you will not be able to use the Pegasus Mail help system unless you download WinHelp from the official Microsoft site (we are forbidden to distribute or make it available ourselves). We are working on developing our own help system to work around this problem. Vista also places new restrictions on applications wishing to perform default system functions (in Pegasus Mail’s case, acting as the default mail program). You will probably not be able to use Pegasus Mail as your default mail program for mailto: URLs under Vista until we produce registration code that complies with the new Vista rules. Finally, Windows now pops up a spurious and annoying warning any time you run an application that does not have a Microsoft Authenticode digital signature from a shared volume. Unfortunately, getting an Authenticode signature is an expensive and arduous process, but we will do so as soon as we can (note that this last issue is not specific to Vista – it will also occur on any Windows XP system where Internet Explorer 7 has been installed).”