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Check the defrag report.
Let us know what kind of error messages it provides, and what version of Windows you are using (XP has a different defragment programme than earlier versions). Important also for us to know is the size of you hard drive and the amount of free space remaining, as well as how long you have had your computer.
Whereof one cannot speak, thereof one should be silent.
Occasionally, there are files that Windows cannot defragment. Once you find out what they are, you can have a better idea of what to do with them. (whether you can delete them, for instance.) Some files that cannot be defragmented aren't going to mess up your computer.