Bad or missing command.com
command.com represents the 16-bit real-mode component of the Windows 9x OS. It is called extremely early in the boot process - immediately after the boot loader io.sys configures the environment by using msdos.sys and config.sys. Because the real-mode environment actually loads the 32-bit graphical environment, a missing or corrupt command.com results in a computer that will not boot. In order to replace or repair command.com, you need a working copy of command.com. This can be found on any other Windows 9x boot disk with the same version. If you don't have another boot disk, you can create one by using the Startup Disk tab of the Add/Remove Programs control panel of any Windows 9x computer. You may also make a bootable disk on another Windows 9x computer by formatting a disk and including the system files. If you do not have a working installation of Windows 9x, then you may find images of these disks on the Internet.
Always use care when downloading files from Web sites that you are not familiar with.
If you have a valid, current Startup disk, you can use it to boot your computer to a command prompt. Then type this:
sys c:
This copies a fresh set of boot files over to drive C:. These files include io.sys, msdos.sys, and command.com.
No operating system found
Although the "No operating system found" error can affect MS-DOS, Windows 9x, and Windows NT-based computers like Windows XP, this section deals with solutions for MS-DOS and Windows 9x. For solutions from a Windows XP perspective, read the next section, "Windows XP Boot Files and Boot Errors."
When dealing with MS-DOS and Windows 9x, this error message means that there is actually a problem with the boot loader program in the boot sector of your drive. Running sys.com on your boot drive should solve the problem. There is also a chance that a formatted floppy disk was left in the floppy drive and it does not have a boot loader program.