If you hover your mouse over the iTunes icon on your Taskbar (the bar at the bottom of your Windows screen), a thumbnail of iTunes should appear (even if it's white):
and if you move your mouse into the thumbnail, you can then tap the X button in its top right corner:
If that doesn't work, you can use Windows' Task Manager to close iTunes:
- to start Task Manager, hold down the Ctrl, Alt and Del buttons on your keyboard and then choose Task Manager from the list that appears
- on the Processes tab, under Apps, select iTunes and then tap the End Task button in the lower right hand corner of the Task Manager