An OS, or Operating System, is what runs your PC. Windows is an OS. If you are trying to save money by not purchasing an OS, then you can try Linux (like Ubuntu) which is free, but you won't be able to play many games.
An OS is always needed. Even your BIOS is an operating system. So is a "live cd" if you boot to (for example) Ubuntu from cd, dvd, thumb drive, etc.