First of all, what is it?  AIN is the system where Windows is notified of a new CD-ROM in the drive.  If found, an appropriate program is launched.  (The type of program would depend on whether it was an audio CD or installation CD, etc.)

Why does it matter?  You may be thinking "but I am not putting any CDs in during the burn process".  This is true.  However, when a CD is recorded, the very first thing burnt is the "Table of Contents".  2 - 20 seconds into the burn, AIN "sees" the new CD and wakes up an appropriate program to try to play it!  

So what if it launches a program?  Actually, it matters.  CD recorders need a constant non-interrupted stream of data to burn on the blank CD.  If that stream is interrupted, the burn fails and you end up with an unusable CD.  When a program launches it steals precious computing power and the burn is interrupted enough to cause a failure.

So how do I turn it off?  Luckily, it is easy to turn it off with MP3 CD Burner.  Just go into the Preferences from the File menu.  Uncheck "Auto Insert Notification".  You will have to restart your computer.  When the computer restarts, it may ask to re-enable AIN.  Just click No.  You can always re-enable it later.

My programs don't automatically pop up when I put in a CD!  This is because you have disabled AIN! :)  To activate the CD in the drive, go to My Computer and double click the CD drive in question.  Otherwise, you can re-enable AIN by going into the Preferences from the File menu.  Check "Auto Insert Notification".  You will have to restart your computer.  (That said, you may have burn failures with it enabled.)

IMPORTANT: If you are using Adaptec's DirectCD for Windows, you must have AIN turned *on*, or some things won't work quite right. The most obvious failure mode is that long filenames aren't shown, but some reports indicate that data on the disc can get trashed as well. This can make life interesting if you're also using a conventional writing application, unless the application is good about turning AIN off before writing.