Another cause to the missing or corrupt Himem.sys is physical memory issues. To identify whether the issue is caused due to hardware issues or due to file corruption, perform the following steps.
Boot the system using a known good boot disk.
If the system boots to the prompt without any errors then the issue is not with the physical memory.
If the error is seen even when booting from the Boot disk then the issue is with physical memory.
Reseat the memory chip or try to boot with single memory chip (if system has more than one chip) or swap the chip to a different slot.
If the issue remains then replace the memory chip.
