ITunes Issue

Hello,
I have a Windows Vista PC with four users set up. When one user has iTunes open on their screen, I get this error message, even though I'm NOT trying to open iTunes manually: 'You cannot open the application iTunes because another user has it open.' Why is it trying to open when not being prompted to? Is there something I can do to stop this annoying msg? It comes up once every 2-3 minutes.

Thanks!