All the above = True.

Remove all your old unused program then do a defrag. Windows defrag will do.

A Defrag arranges all your files that is spread into multiple segments into a more accessible way next to each other so that your hard disk don't need to fetch other segments of the same file in 10 different places on your hard drive.

It does speed up your machine but only if you are working with mostly large files that is spread into multiple segments...