I'm not much of a windows user anymore so I don't know of any tools on widows that might have this functionality. There could be a market for an email app. or browser plug in that would do this for you in windows.
Two of the limitation of a computer reading to you is one, speech is a lot slower than reading, and two the voices can be quite monotonous making it difficult to focus on while multitasking.
I've heard of some really interesting software for the blind that reads the contents of whatever they're "observing" at an accelerated pace. Apparently your ears can adjust to an extremely accelerated rate of speech making the process of hearing the text on your computer closer or even faster than the speed of actually observing it. Imagine writing software without a monitor! If you could get people to learn this skill we might be able to read our RSS feeds (or emails) much faster!
Something somewhat related to this is voice recognition. There is a lot of good software out there that can translate what you're saying into words, it should be hooked up to the operating system so I don't have to use a mouse and keyboard so much! When I visited MSFT on a job interview I talked about this with a guy who worked on related things. He felt that the technology was still a long way away. Prove him wrong!
On a related note, I was surprised to discover that "say" works over remote logins. Try "ssh some-mac say hello world". Good for Halloween.