Is there an ac3 codec I can obtain that once installed on my pc will allow me to play ac3 files from almost any player/editor (ie. windows media player, mydvd, vlc, soundforge, etc.)

AC3 is AC3. Any DVD player software will playback AC3.
No Sony product will open an AC3 file, as a general rule. AC3 is not supposed to be edited by virtue of Dolby's copyright, although there are several tools that will open AC3 in an editable application.
Anyway, most any media application such as MyDVD, Windows Media Player, WinDVD will open video files containing AC3 audio.

AC3Filter or ffdshow (recent versions)

If you haven't find it yet here is the link:
http://ac3filter.sourceforge.net/

It made possible playback of DVDs with AC3 audio with otherwise uncapable Power DVD 5 OEM version.

Sony DVD Architect will open and play AC3.

Azid will convert AC3 to WAV. Download from www.doom9.org It's a command-line program but there are a couple of GUI's for it around.

problem with dvdarchitect is that it has some issues with metadata..

it also does a stereo mixdown (for preview purposes) of 5.1 projects, it doesnt actaully allow u to edit the file.

There are no "actual" programs or editors which allow u to edit an AC3 file as DSE mentioned, Dolby licensing restrictiions and their strict adherence requirements (not to mention their legal team) dont like the idea of people manipulating a format which is owned by them..

however if youd like top create seperate wav streams from a 5.1 file, you can so with BeSweet