Substitute your Windows username for username in my example line above. The reason I had to do this is for whatever reason, GMail Notifier was unable to override Outlook when I told it to make GMail and Chrome my default mail handler. Probably because Windows Vista is a PITA and Chrome's support by other programs, even Google programs, is still a bit funky.
For those who are curious about how this works (and don't know immediately by looking at it,) here's what's going on... The registry key's data is exactly the same as something you'd enter in the command line. In fact, when I was experimenting with the path and how to include the address I wanted Chrome to open, I used the command line. Then I copy-pasted the succesful command into the field in regedit where I could change the value of the key. %1 is a placeholder for the piece of data being passed to the command. I had to play with that a little to make it work because I wasn't sure if the mailto: part was included automatically or not. It turns out it is, which is why you don't see it in the registry key above. Chrome is smart enough to open a tab, not a whole new window, when it gets started this way, which is nice because it saves learning more about how to operate it from a command line.
In the spirit of credit where credit is due, I figured out a lot of this based on someone else's registry key. Check out Shonzilla's blog. I think that perhaps his version would work on a previous Windows version?