Your EZCD software is fine; I would suggest increasing your laptop memory up to 128, but neither of these will impact the sound quality. EZCD writes the image, but what software are you using to actually record the .WAV file with?
While I certainly know what an ADAT is, I have not worked with one. I'm going to guess that you are "bouncing" tracks down to a stereo left/right signal first, then running from an (analog) stereo output into your mixer, then from the (analog) Main L/R Outs of the mixer to the (analog) line in of the sound card.
First step - Your mixer has a monitor out connection. If you can plug in headphones or a amp/speaker to the mixer and it sounds OK, then you can assume that the signal is getting to that point OK.
Second step - as the sound is entering your laptop, hopefully whatever recording program you are using has some form of monitoring. recording digital signals is different than analog in that you NEVER want digital recording to peak over 0db. So if your level is too hot you will get digital distortion.
Remember that Windows has its own volumne control in addition to any controlls in your program, you may need to adjust this.
If the levels are ok, check the cable from the mixer to the laptop.
It is unfortunately possible that the laptop just has a poorly wired line-in connection, if so there may not be much you can do.
Thats all I can think of at the moment...