If MP3 is your codec of choice, and you don't really care about fancy GUIs and unneeded crap, LAME MP3 is your best bet. LAME produces the best MP3 files at any bitrate (compared to other MP3 codecs) and it's completely free.
You can get it here: http://www.rarewares.org/mp3-lame-bundle.php
Then download LameDropXPd: http://www.rarewares.org/mp3-lamedrop.php
LameDrop is portable, so you can store it anywhere, create a shortcut or pin it to the taskbar. From there, you can set encoding quality, output dir and tags by right clicking the window. Once you're set, it's only a matter of dragging and dropping the files into the window, and letting the program do its stuff.
Keep in mind, though, that as of my last use of LameDrop, I think I remember that the conversion can only go either WAV-›MP3 or MP3-›WAV, so LameDrop is a good choice if you rip CDs or edit them in WAV format then do a compression to MP3.
Some of the more detailed (but still simple) converters I can think of right now and recommend are: http://www.freemake.com/free_audio_converter/
and fre:ac - free audio converter