Summarize:

By running the defragmenter, you reorganize the file data and make it easier for your computer to find what it's looking for and less likely to freeze. To access this utility, go to Programs>Accessories>System Tools>Disk Defragmenter.  Everyday processes scatter your computer's file data physically on the hard disk which then causes the computer to take longer in retrieving them. Defragmenting corrects this issue and may take anywhere from 10 minutes to several hours depending on the size of the disk. Read Defragment-a-Disk-on-a-Windows-Computer or Use-a-Disk-Defragmenter for more detailed information. Sometimes uninstalling and then reinstalling applications can help them to run faster and your Windows operating system is no different. Formatting your hard disk deals with erasing all the files on your primary disk in preparation to reinstall Windows using your original install disc(s).  Be sure to back up your data first! Formatting means everything on your hard disk will be deleted. On Windows 8 and up, you can use Refresh action and Windows will reinstall itself, without any DVDs and deleting your data. Check out Reinstall-Windows-7 or Reinstall-Windows-XP for more details on the reinstallation process. If your system only just meets the minimum requirements for its current OS, downgrading may help improve your performance.Downgrade-Windows-8-to-Windows-7 details one specific operating system downgrade.
Defragment your hard disk. Format your disk and reinstall your operating system. Downgrade to a lower operating system.