Firefox has a Speak It extension which can read aloud web pages or highlighted text. One application is read aloud the news in the background, say from an RSS feed in one tab, and to continue browsing the web in another tab. However, with the limited options available in Speak It, it's too hard to do this in Firefox at the moment.
This extension works in Windows XP, which comes with a default voice for text-to-speech synthesis, known as Microsoft Sam. If you're tired of Sam, you can get two additional voices from Microsoft, Mike and Mary, from the Speech SDK 5.1 page. The SDK also includes a Sample TTS Voice but I can't get it to say more than, "Blah blah blah," in Speak It.
The hassle is that you have to download the 68 Mbyte SpeechSDK51.exe. That SDK page suggests downloading Sp5TTIntXP.exe but this file contains an MSM file that you can't install; developers include MSM files into a software distribution using a development tool such as Visual Studio. Once you've installed the SDK, you can select a different default voice for your computer in the Speech control panel applet.