I'm away on vacation now with only a Mac at my disposal so haven't had a chance to try this, but Google has just announced an Instant Messaging solution called Google Talk - it requires a download. From the site:
They say talk is cheap. Google thinks it should be free. Google Talk enables you to call or send instant messages to your friends for free–anytime, anywhere in the world. Google Talk offers you:
* Choice: Get in touch how and when you want to–over email, IM or a call
* Quality: Talk through your computer but hear your friends as if they were in the same room
* Convenience: Your Gmail contacts are pre-loaded into Google Talk so inviting or talking to your friends is just a click away
Google Talk is in beta and requires a Gmail username and password.