many ways to make gmail your default mail app.
You can edit the Registry. fun fun, unless you know what you are doing, avoid this one. I know its "HKEY_LOCAL_MACHINE\SOFTWARE" some place where "mailto" is listed.
or easier..
Install Gmail Notifier, or Google Talk, they have options in them to make all "mailto:" links open up webmail
Affixa is another software that will help.
If you google it, there are a million hits.. try this link, under google forums.
http://www.google.com/support/forum/p/Talk/thread?tid=5630cbddac6bbb47&hl=en
