Try dbPowerAmp Digital Music Convertor, here - http://tinyurl.com/5hewx
You need to install a codec addon from here - http://tinyurl.com/4tftn (two if you want to be able to convert to AAC as well)
Just to verify - the one you would need to be able to convert from .acc to .whatever would be the first one, "mp4 & AAC Decoder", and the one you would need to convert from .whatever to .acc would be the first on up from the bottom, "AAC Codec"
When it asks you to locate the encoder (asks for the location of the addon the first time you use it) you do need to point it to the right place, do a Windows search to find the file it's looking for.
iTunes seems to be creating and downloading files as .m4p. Within iTunes you can't convert to .acc as it says they are protected. How can I un-protect? Is tere a codec addon to convert from m4p to a.n.other format? Cheers for all your help so far.