ITunes VERY slow to respond

Hello! We have Windows Vista. ITunes was working fine until a couple of weeks ago (my kids just told me). Now, it takes up to 10 minutes to respond to each request, from initial opening to clicking on any song choice. This makes downloading music virtually impossible. I also tried to upgrade to ITunes 8.1 and there was an error. I have the files downloaded but they will not install (error says that it cannot locate file %app%).

Can anyone help?