There are a number of free text-to-speech applications which can read out emails or documents. They will leave out lots of visual information such as if text is bolded or an email has an attachment, and are therefore not very useful if you need to use a computer but can't see the screen. Software that reads out this additional information is called a screen reader. This page discusses free screen readers.
Most screen reader users do not use a mouse. Windows has keyboard accessibility built into it, and when you use a screen reader you make use of this. A screen reader will have its own keystrokes as well, for instance you would expect a keystroke to read the entire content of a document on screen. Screen readers often have their own "modifier" key - a key used in conjunction with one or more keys to perform screen reader commands - to ensure the screen reader commands do not clash with those of Windows or the current application. Common screen reader modifier keys are the Caps Lock key or the Insert key.
The voices that free screen readers use are often not appealing at first. You can obtain cheap - but not free - alternatives that are better sounding. However, voices which are not appealing at first may grow on you, or they may have other benefits such as responsiveness or the ability to remain understandable at high speech rates.