Since you posted in the Windows ME forum, the literal answer is "Windows ME".
But I suspect you didn't elucidate your issue and my bet is that you have no OS CD. If this is so, you don't have an OS to reinstall. Depending on your circumstance you pester the supplier of the machine for the OS unless they installed it for free then they have no right or need to supply it. If you lost the CD, but still have the registration and if it's not an OEM, then Microsoft still can replace some CDs. Or you look on Ebay or elsewhere for your OS CD.
This is a lesosn many learn the hard way about the need to obtain and keep your OS CD.
i am unable to get my computer out of safe mode so i have been told the my OS is badly damaged so it needs to be reinstalled but what is the OS?