As pcs get older and files are created, deleted, moved, and copied the data that makes up those files may not be in one contiguous space on the disk. This will eventually cause the pc to take longer and longer to access the file. What a defragmentation program does is re-arrange the files on the disk so that all of the files pieces are contiguous. This usually results in faster disk performance.
The program has a very simple interface. It shows a list of local hard drives. If you select a drive and click analyze it will show you a graph of the drive with color coded squares indicating file fragments, free space and contiguous files. If the graph is mostly red it most likely needs to be defragmented. This is done with a simple click of the "Defrag" button and some time (possibly a lot of time). Depending on the size of the drive and the amount of fragmentation it could take hours to finish. For example I tested the software on an older laptop with a 20 GB drive and it ran in about 1.5 hours. For those of you that have much larger drives you may want to run this one over night. As always BACKUP your data just in case. (for backup tips see my 7 Ways to Backup Your Data post).
Please post a comment if you have any other good Disk Defragment programs I can test.