Re install your OS using following steps:
In case you dont have a disk you can install windows using a USB flash drive by following these steps:
Step 1- Download ISO image of Windows from: windows.microsoft.com/ website
Step 2: Download YUMI from: www.pendrivelinux.com/yumi-multiboot-usb-creator/ website?
Step 3- Plug in your USB in pc.
Step 4: Run YUMI and follow the on-screen instructions by selecting the appropriate operating system.
Step 5: Give path of the ISO Image and select the USB drive to make boot-able disc
Step 6- Plug in the USB onto the system on which you need to install the windows.
Step 7- Turn on the system.(Change the boot preferences to USB if the system do not automatically start booting from USB).
Step 8- follow the on screen instructions to complete the windows installation. Thanks