Go to your control panel (Start >> Settings >> Control Panel) and double click on the Speech icon. Switch to Text To Speech tab and select your options. you will be able to change the voice as well as other speech options like the talking speed. You must have Text-To-Speech engine installed on your system for these options to be available. Please visit this URL if you want to download and install the Microsoft Text-To-Speech engine for use with Talking Reminder.
http://www.softwareriver.com/html/speech.html
NOTE: Windows XP users have the speech engine available or already installed on their computers. However - there is limited number of voices available. To add "Mike and Mary" voices to your speech capabilities, please go to this URL http://www.microsoft.com/speech/download/sdk51/