Keep Windows as a another OS is easy on Linux system. Just insert the setup CD/DVD into ur cd/dvd drive and follow on-screen instructions, but be careful at the time of partition, con't choose system proposal/auto partition, rather select Custom partition where u can divide how much space leave for Linux, while Windows is already installed.

Most Linux distributions will install GRUB upon completion of installing Linux, and it will usually add Windows to the bootloader automatically without additional configuration to the bootloader. Ubuntu is very user friendly, and can even be installed within Windows, so usually, dual booting isn't very challenging or difficult when things go right during the install.