If you want to reinstall windows 7, please follow these steps :
1-First you need to backup all you data (pictures, files, videos, ect...)
2-Shut down your computer, put the Windows 7 disc in the CD/DVD slot and start it up. All being well, it should boot from this disc. If it boots to Windows normally, you’ll have to use the BIOS menus to make your computer boot from a disk. If you don't have the Installation disc you can download windows 7 from this link and burn it into a blank dvd :
3-Windows Setup will start and, on the first screen you should specify your preferred language, time and currency format, and the nationality of your keyboard. You’ll also be shown a screen on which you’re required to agree to the licence conditions : http://cdn2.pcadvisor.co.uk/cmsdata/features/3535423/How-to-reinstall-Windows-7-Step-4_thumb.jpg
4-You’ll now be asked “Which type of installation do you want? Please Choose the Custom (advanced) option.
5-Next you’ll be asked "Where do you want to install Windows?". Sometimes just one partition will be shown, in which case just click on Next. If multiple partitions are displayed, select the first primary partition (usually the largest or with lettre C: or (0)) before clicking on Next. : http://cdn2.pcadvisor.co.uk/cmsdata/features/3535423/How-to-reinstall-Windows-7-Step-6_thumb.jpg
6-click drive options (advanced) and choose format, wait a few minutes and click on next.
7-Windows 7 will now be installed and progress will be reported in the list of actions and the progress bar
8-Next, you’ll be guided in setting up a user account.Follow the onscreen instructions to finish the installation.
Additional Information (tutorial video) : https://www.youtube.com/watch?v=mME9VLT7-xw
Hope this help, let me know if you need further help