To do this, you'll need to start, or boot, your computer using the Windows 7 installation disc or USB flash drive.
1. Turn on your computer, insert the Windows 7 installation disc or USB flash drive, and then shut down your computer.
2. Restart your computer.
3.Press any key when prompted, and then follow the instructions that appear.
4.On the Install Windows page, enter your language and other preferences, and then click Next.
If the Install Windows page doesn't appear, and you're not asked to press any key, you might need to change some system settings. To learn how to do this, see Start your computer from a Windows 7 installation disc or USB flash drive.
5. On the Please read the license terms page, if you accept the license terms, click I accept the license terms, and then click Next.
6.On the Which type of installation do you want? page, click Custom.
7.On the Where do you want to install Windows? page, choose the partition that you want to install Windows 7 on, or click Unallocated Space if no partitions are listed, and then click Next.
If a dialog box appears saying that Windows might create additional partitions for system files, or that the partition you chose might contain recovery files or other types of files from your computer manufacturer, click OK.
8. Follow the instructions to finish installing Windows 7, which include naming your computer and setting up an initial user account.
i start to say my problem from very begining.i have installed windows 7 and from so long it was running fine but after attacking some virus now it has created problem so i tried to reinstall windows 7 but all other partition had not any option for formatting or delate and partitions are showing dynamic.i want to completely format my hard disk but now no os is installed in my hard disk