Audacity and lame -- you can then run this script.
It takes a text file and uses SAPI to first convert to WAV format -- then lame to convert the WAV to MP3 format.
SAPI, I think, is available on most Windows based PCs. On Windows XP it comes with 'microsoft sam' -- with later versions of Windows 'microsoft anna' is the default voice (much better).
Download AutoIt script and compiled binary:
This version uses 'b -320' lame encoding for better quality MP3.
The file downloads to TextToMP3SE.exe. This is a self-extracting file. When run it creates a folder called 'MyItzyBits' and extracts TextToMP3.au3 and TextToMP3.exe into this folder. The .au3 file is the script shown in the image here -- the executable is the compiled binary. To use -- drag-drop text files atop the executable TextToMP3.exe.