Nice links...

FYI: Defragment places files and data in order. For example, if you save a text document as example.txt, and later install some program such as IWantToBeSmart.exe. Your HDD puts things in order by time (first come - first serve). Right now your HDD reads:

example.txt
IWantToBeSmart.exe

Now lets say you want to change something in example.txt, and you save it. You now have:

example.txt
IWantToBeSmart.exe
example.txt

What disk defragment does is place everything back in order with the files that correspond.

example.txt
example.txt
IWantToBeSmart.exe