First, the file "C:\Windows\WIN.COM" must exist and, for Windows 98/98SE it must be exactly 24,791 bytes. If it doesn't exist, or it is the wrong size, follow the directions in the "Resolution" section of the page I linked to above to recreate the "C:\Windows\WIN.COM" file.
Next, the last line in the AUTOEXEC.BAT file, (on the disk you're using to boot the system into Windows with), needs to point to the exact location of that WIN.COM file. That last line should read:C:\Windows\WIN.COM
(Note: It doesn't matter if that line is UPPER, lower or mIxEd case).
Also, while you're viewing/edting the AUTOEXEC.BAT file, check to make sure that "C:\Windows" is included in the "PATH=" statement. The line should look something like:path=c:\windows;c:\windows\command;c:\whatever...
If a "PATH=", (or maybe a "SET PATH=") line doesn't exist, add the following line just above the WIN.COM line:path=c:\windows;c:\windows\command