I had tried every suggestion I could find on google to fix the stuttering apart from re-installing Windows XP until I found a solution that works for me. Maybe it will work for you.
I've got Windows XP Service Pack 3, 2 gigs of ram and it was stuttering badly.
To fix this all I had to do was go to Task Manager by pressing ctrl-alt-del, right click on iTunes.exe under the Processes tab and set priority to Above Normal.
You will have to do this every time you use it unless you create a .bat file to make it happen automatically.
This is how I created the bat file (below). You will need to change the bit after "abovenormal" to wherever you have saved iTunes. Mine is saved on my P drive in the itunes folder, but yours will be saved somewhere else.
I opened Notepad and typed the following:
start /abovenormal p:/itunes/itunes.exe
Then Save As itunesloader.bat
Now all you have to do is double-click on that bat file to open iTunes in above normal priority mode every time.
Hope this helps some frustrated people out there.
Damien