Summarize this article:

It’s at the top of the window. It’s in the “Defragmentation” group. This opens the Disk Defragmenter window. Your computer will analyze that disk drive's files and folders to determine whether defragmentation is necessary. The drive will now be defragmented. This process may take several minutes to several hours. If you see a message that says the drive does not need to be defragmented, there’s no need to take any action.

Summary:
Click the Start menu and select My Computer. Right-click your hard drive and select Properties. Click the Tools tab. Click Defragment Now. Select the drive you want to defragment and click Analyze. Click Defragment if recommended by the tool. Click Close when you’re finished defragmenting the hard drive.