You have a few options. Unfortunately without a Windows disk, you can either contact your computer manufacturer and see if you can purchase a installation DVD for the version of Windows that you are currently running. You could also download the ISO file from the website below and use burning software like IMGburn to burn the ISO file to a disk.
You will also need to make sure that you have a product key on the side of the case to reactivate Windows. If you do not see a product key, try using a keyfinder on the PC like the one below.
Feel free to ask me for more information and help
Windows Download:
http://best-windows.vlaurie.com/boot-disks.html#full
ImgBurn: http://www.imgburn.com/
Keyfinder: http://www.magicaljellybean.com/keyfinder/