Problem when Windows restarts

I have a rather old Compaq Presario 1620 laptop with Windows 98. I seem to have a problem warm booting the computer. It does not boot Windows but gives me an error at a DOS prompt:

HIMEM.SYS is missing.
Make sure that the file is in your Windowsdirectory.

I have checked my Windows folder, and the file HIMEM.SYS is there. The only way I can restart my computer is to do a cold boot. Could someone help me with this please. I would appreciate it very much. Thanks.