Deleting/erasing all my files from my desktop computer

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I am moving from a desktop to a laptop; I have backed up and transferred all
files, Outlook and IE favourites. I now want to sell my desktop/hard
drive/monitor but before doing so, I need to remove all traces of my files
from the hard drive. I know that deleting files and emptying the trash does
not erase the files and files could be reconstituted. Can someone let me know
what the process is for doing this.

Once I have thoroughly cleansed the hard drive, do I need to also remove
Windows as it's licenced to me?

Many thanks.
.

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