Open up the Registry Editor and navigate to HKEY_CLASSES_ROOT -> mailto -> shell -> open -> command -> (Default)
Right click (Default) and click Modify
Change the value to:
"C:\Documents and Settings\USERNAME\Local Settings\Application Data\Google\Chrome\Application\chrome.exe" --app=https://mail.google.com/mail?extsrc=mailto&url=%1
(if you've installed Chrome somewhere else, change the path to where Chrome is installed)
This will pop up a new Gmail 'compose message' window with the email address filled out.
This method requires cookies to be enabled and the option for "keep me signed in" to be checked on the gmail.com login screen