Normally, the cause of that message is a corrupt command.com file. To replace the file, get into BIOS by pressing and holding either the F2 or F10 key during startup, highlight "Advanced BIOS Features" (or similar) using the arrow keys, press Enter to select that option, scroll down to "1st Boot Device" (or similar), configure the floppy drive as the 1st boot drive by pressing either the Page Up or Page Down keys, press the Esc key until you're back to the main menu, insert a Win 98 boot floppy into the drive at this time, highlight "Save And Exit Setup" (or similar), press Enter, type Y at the prompt, the computer will restart and boot from the floppy, at the menu that appears select "Start without CD-ROM support", press Enter, type "sys c" (without the ") at the A:\> prompt, press Enter, when the A:\> prompt reappears remove the floppy from the drive, restart the computer and go back into BIOS, configure the HD as the 1st boot device, save the change, and restart the computer. It should now boot up into Windows.
If you do not have a boot floppy, then you can make one here on another system: