Computer shuts down only for one user

Hello,
I have a small business. One of our PC's is used by 2 employees (1 on 2 days per week and the other on 3 days per week). The one who uses it twice per week is constantly complaining that the computer will automatically reboot in the middle of her using it. It never happens when the other employee is logged in and I have never been able to witness this happening. If anybody has any idea what might cause this please enlighten me. We have tried the typical CC Cleaner and Virus scans.

Thanks,
Dan