I don't know how long this has been available, but I just found it + think it's a very useful little programme to sit in the taskbar.
Everytime you receive an unread e-mail to your gmail account, you get a little message pop up on the bottom right of your screen. You can see who it's from + the subject title.
There's an option to make your gmail use mailto: links in webpages http://www.winxpnews.com/rd/rd.cfm?id=050531TI-Gmail_Notifier&mid=8620750410228695
CLICK HERE - GMAIL NOTIFIER
I don't usually like having too many icons in the taskbar, but think I'll be keeping this one on there.
It only works with Windows 2000 + XP at the moment.