Yes you do need to reinstall the OS. There are other steps to take as well. Your motherboard/video card drivers will need to be reinstalled after you install the OS. If you've built the computer yourself, find the drivers disk that came with your motherboard and/or video card. Install motherboard drivers first. If you don't have these, or you purchased the PC already built, Visit the manufacturer website on another computer to download all drivers. Put them on a disk and install them once the OS is done.
1. To install OS, insert OS disk when your computer is powered on (so you can open your disk drive). Restart, let windows disk load.
2. Press any key when prompted, and then follow the instructions that appear.
3. On the Install Windows page, enter your language and other preferences, and then click Next .
4. accept the license terms
5. 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 .
6. Follow the instructions to finish installing Windows 7 , which include naming your computer and setting up an initial user account.