Just did some quick checking trying to find some alternatives, extra voices etc.
SAPI 5 Voices
Microsoft Sam is a commonly-shipped SAPI 5 voice. In addition, Microsoft Office XP and Office 2003 installed L&H Michael and Michelle voices. The SAPI 5.1 SDK installs 2 more voices, Mike and Mary. Windows Vista includes Microsoft Anna which replaces Microsoft Sam. Anna is designed to sound more natural and offer greater intelligibility. Several multilingual voices are also included in localized versions of Windows Vista. Microsoft Anna is also installed on Windows XP by Microsoft Streets & Trips 2006 and later versions.
http://en.wikipedia.org/wiki/Speech_Application_Programming_Interface#SAPI_5_Voices
I think the Vista voice Anna is a "Realspeak" voice which after listening to several comparisons from different vendors - Realspeak sounds (to me) like the most clear voices available.
Check out Freeware DSpeech - http://dimio.altervista.org/eng/
http://www.cepstral.com/demos/ (Try out the Character voices - I love the Marine "Shouty")
http://www.cepstral.com/downloads/ "several" voices for purchase $29.99
PenfriendXL - a comprehensive literacy aid which can boost confidence and increase reading and writing skills across the age and ability range for: those with dyslexia, physical disabilities including visual impairment, anyone learning another language.
http://www.penfriend.biz/
http://www.penfriend.biz/products/download_voices.shtml
A compact open source software speech synthesizer for English and other languages. (Additional voices and espeakedit program and the phoneme data).
http://espeak.sourceforge.net/
If you want to get only the Mike and Mary voices redistributable for Windows XP, download Mike and Mary redistributables (Sp5TTIntXP.exe).
http://www.microsoft.com/downloads/details.aspx?FamilyId=5E86EC97-40A7-453F-B0EE-6583171B4530&displaylang=en#Instructions
CLiCk, Speak is an open source, freely available extension for the Firefox web browser. It is part of the CLC-4-TTS Suite of products, it features a mouse driven interface, and it reads web pages - hence its name.
http://clickspeak.clcworld.net/
Sorry it's all just thrown together, but I'm writing this from bed on my laptop (got the flu) - and I can't be stuffed formatting it any better. ;)