After the reminder, I checked and found that there's an Autoplay tab on the Properties window of the CD drive if you enter from My Computer. On that tab, there's a pull-down menu where you can select a CD type (music, DVD movie, etc.), and after you select a type, the window below it shows different actions the PC will take when you insert a CD of that type. One of those actions is "Take no action". That oughta do it. I'm using XP Pro and after setting this window, I never had to download PowerToys or TweakUI or anything, and the setting sticks - my DVD drive never autoplays, whether I put in a game disc, music, whatever. Hope that helps.
Jason