Many people defragment their disks on a regular basis seeing it as the panacea against slow computers. Unfortunately, the impact is most of the time fairly limited.

First, you must have at least 15% free space on your disk for the fragmented files to be temporarily copied before being consolidated.

Second, although degragmenting will improve performance, it will always be negligible compared to a memory upgrade or bloatware/malware removal. Also try to limit the amount of programs automatically starting with Windows and thus constantly running in the background even though you are not using them.

So when is defragmentation really effective? Although it won't do any harm, the time when you want to really run it is after you uninstall a lot of programs or delete/transfer a large amount of data. A typical example is copying photos to an external hard drive when your disk gets full. This will leave huge gaps on your original drive resulting in newly saved files being scattered across the drive (ie fragmented)