Hello and welcome to justanswer, My name is ***** ***** i will be assisting you today,I'm sorry about the issue, please follow the steps below :
Turn on your computer, insert the Windows 7 installation disc and then turn off your computer.
Restart your computer.
Press any key when prompted to do so, and then follow any instructions that appear.
additional information : https://www.youtube.com/watch?v=guLKA-1gjfg
If you don't have the windows 7 disk, please download the iso image of windows 7 from : http://artechies.blogspot.com/2013/01/kumpulan-direct-download-link-windows-7.html
once you download it, Insert a blank CD into your computer and burn this image to this CD.
If you need further help, please let me know and i will gladly continue to help