I found this answer on a Google forum that has worked me. In the footnotes it mentions 'googleapps.exe' - I don't have this and it has still worked. Simply follow the instructions below but close down all applications before making changes to the Registry. Also I saved the existing value just in case it didn't work.
Simply type "run" in your search bar, then type "regedit" then travel to:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\mailto\shell\open\command\
edit (double click) "(Default)" to:
"C:\Program Files (x86)\Google\Google Apps\googleapps.exe" --domain= --mailto.google.com="%1"
That's it! Save and close it and it should work beautifully!
Using this method prevents you from having to download the GMail Notifier, which for those of us with GTalk don't need since it does it for us. I'm not sure why Google can't solve this issue easily.. i've heard Google Apps haven't been tested fully on Windows 7 but it's obvious the same tag works with it.
Note: The only thing with this solution is you need to have the googleapps.exe file on your machine. I believe I got it with my free GooglePack from their site which has now been discontinued. I tried searching the net for a way to download it but weirdly enough it seems it's reserved only for Businesses now and there is no download link available from the web because everyone who has it streamed it using the google updater.. Odd. Anyway good luck!