Over time you write a lot of 'stuff' on to your computers' hard drive. Windows does it's best to put a file into a single sector of the hard disk, but sometimes this is just not possible. The file is then split into several sections and placed in the next available sector of the disk. This is called a fragmented file. Over time, more and more files we become fragmented as they shrink and grow, get deleted or moved. Fragmented files take longer to read than the non-fragmented brothers. So, the more fragmented files you have, the slower your PC will get...until you can stand it no longer and run defrag. In steps Diskeeper.