Outlook 2000 won’t open

I am having a problem with a client who cannot open their Outlook 2000. It has only started the last two weeks. The only way she can open it is to restart the computer. The pc is running Windows xp with Office 2000. When clicking on the icon, the hour-glass comes up and then it disappears.
Could this be virus-related?

Look forward to a response.
Mil