It's a very strange thing I have never seen. But try to use Windows XP+SP2 with your protool.
I didn't seen this problem as While I was using Windows XP+SP2 and protool, I moved to winCC flexable. And then I moved to Windows XP+SP3 with winCC flex. No more work with protool. Any way, just try to use Windows XP+SP3 or try to use WinCC flexable. You can migrate the protool project to WinCC flexable project easily and without instaling Protool.