Sigh.
If you have a machine running DOS, Windows 95, or Windows 98, go to a command prompt and type:
format a: /s
The /s option writes the (bootable) system files.
If your machine is running WinMe, you can't do this and must create the disk from Add/Remove programs.
If your machine is running Windows NT, Windows 2000, or Windows XP, you can create a bootable floppy, but it will have to point to an existing operating system on the hard drive. This is useful if your startup environment is hosed but the OS works, like if you added a disk or changed a disk controller or something. In your case, this is probably not useful, but for the sake of completeness, to do this, format a floppy disk under NT4/W2K/XP (do not do a quick format) and copy boot.ini, ntdetect.com, and ntldr to the floppy. You may need ntbootdd.sys depending on your configuration, and you may need to edit boot.ini appropriately for the target machine.