Defragmenting the hard drive in linux

The short answer is that you don't need to defragment on most Linux file systems. Basically, they don't fragment. That is, files are kept whole on the harddrive. FAT filesystems tend to break files up and cubby hole bits of files wherever they can stuff an extra 1024 bytes. Eventually, the files on the system are fragmented so badly that they need to be "cleaned up" to prevent wasted time putting it all back together.
If you are going to install Linux as a dual boot along with a windows partition, you will need to scan disk and defragment that partition before creating a Linux partition and installing. Defragmenting the windows partition moves everything to the top of the drive, making it safe (or safer) to re-size the windows partition to make room for the Linux partition.