Open Windows Explorer and search for a folder called Microsoft Office. Whoever you got the computer from may have stored the installation files on the computer. If you find the folder, open it and look for a file called setup and double click it. It seems as if the installation was not finished when you got the PC.
If you don't find the folder, you will need to borrow a disc from someone, or go back to where you got the computer from and ask for the disc.