Man, I've been looking for an app that will do this for YEARS!  It's been a while since I used a dedicated mail client like Outlook or Thunderbird, since I mostly just use a single Gmail account for almost everything these days.  The trouble comes when you click on an email link on a web page, or from a PDF or other document ... Windows will try to use whatever it's been told the default email client is (or Outlook Express if you haven't configured a different one).  But that's not what I want - I want to use Gmail (which does have an "offline" gizmo in developement, but not finished yet.)

If I remember correctly the Yahoo! Toolbar will give you the option of making your Yahoo! email account the Windows default - but I also remember problems trying to get it to give up this behavior too!  (It also requited installing the Yahoo! Toolbar, which I'm not crazy about...)  Affixa is a free program which will allow you to use either Gmail, Yahoo! Mail, or your default local client (Outlook, for example) as the application that will launch when you click on an email link.

With this free version, you can set up ONE of these accounts to be the default - with the registered version (which cost "2-pounds/year", if I'm reading it right ... or maybe that's Euros now - beats me) you can set up all of your email accounts and programs and choose which to use when you click on an email link.

Until Google gives me some better toys, this will do...