You can't uninstall Windows 7, but you can reinstall your previous operating system if you have the original installation disc. For more information, see Installing and reinstalling Windows 7.
However, reinstalling an earlier operating system not only replaces Windows 7, but also deletes all of the programs, files, and settings on your computer.
Before reinstalling your previous operating system, you should save your files and settings to a storage device, and then save them back to your computer after the installation is complete.
Because your programs will also be deleted, you’ll need to reinstall your programs using the original installation discs, or using the setup files if you downloaded the programs from the Internet.
If you are running a multiboot system, you might be able to safely delete or reformat the Windows 7 partition and reclaim that space for use with the earlier version of Windows. For more information, see Uninstall Windows 7 on a multiboot system.