What you are looking for is called a screen reader. Windows includes one as part of the accessibality tools, or you can purcahse a third party one.
In Windows Vista it’s available in the control pannel under Text To Speach.
In Windows XP it’s available in the control pannel under Speach.
Alternately, there are many programs that do this available on the internet– some of which are free. Try these:
<a href=”http://www.nextup.com/TextAloud/”>Text Aloud</a>
<a href=”http://www.google.com/search?q=text+to+speech+software&rls=com.microsoft:en-us&ie=UTF-8&oe=UTF-8&startIndex=&startPage=1″>Here</a> is a search that yields possible results.