HIMEM.SYS is missing

This novice is baffled: When I do a soft boot, Windows 98 will not start. Instead, the DOS screen appears informing me that “HIMEM.SYS is missing. Make sure that the file is in your Windows directory.”

The file is in the Windows directory. I have renamed, copied and restored the file and still get the same problem on a soft boot or restart.

When the system is powered-down and powered-on, Windows starts as you would expect.

How do I fix the problem?