That explains it all. If you press and release "Ctrl+Alt+Del" key on your keyboard while in windows. This will bring up a window called "Task manager". Click on Processes and you'll see which process is taking allot of resources. Click on the performance tab and it will show you how good your processor and memory is doing.
Saying all that, the main problem I can see with your itunes is lack of memory. Upgrading it to 1GB or 512MB would make a difference.