Hard Disk Fragmentation
Operating systems, applications and users constantly create, open and close files. The file systems do not necessarily place data on the storage media as a single block when they write a file to disk. As a result, over time, the files on the drive can be spread across different sectors of the hard drive. When files are spread over different sectors of a hard drive, the drive is said to be fragmented. This fragmentation can seriously degrade system performance, as each time a file is opened it must be gathered from several places and stored in memory.
Disk Defragmenter can be used to analyze the amount of fragmentation on a disk and can rearrange the disk, placing the files in contiguous blocks. The newest versions of Disk Defragmenter have logic that makes them aware of the way the operating system reads executable and dynamic-link library (DLL) files, so they can place clusters in the order they are read. Both of these operations can significantly improve system performance. This utility should be run at least once a month.
You can schedule the defragmentation to take place when the computer is not in use though screen savers and other programs that run in the background should be disabled during defragmentation.