The operating system is Windows 3.11 which is the operating system that came before Windows 95.
I noticed that information about Eraser 5.7 says it works for Windows 95 and then other operating systems that are more recent than Windows 95, so I thought that perhaps 5.7 Eraser does not work for computer with a Window 3.11 operating system.
I've just seen the computer, which is at my 92-year old mother's house. Earlier someone had installed Window 98 on the computer, so the system is no longer Windows 3.11. Thanks for the earlier advice, but I guess I will be able to use the 5.7 version of Eraser.