If you want POP access to GMail, there's PGtGM. If you just want a notification, there's now an official GMail Notifier:
Philipp Lenssen writes "After several unofficial, screen-scraping Gmail utilities, Google now released the official Gmail Notifier (Beta) for Windows. It will sit in the Windows tray, alerting you of new emails in your account (if you are lucky enough to have one already). Additionally, the Gmail Notifier can connect 'mailto:'-links in web pages to Gmail."