Folder

A folder comes up on my desktop everytime I start my computer. The message within the box states this folder is empty. Does any one know how I can stop this please. Also I want to give my computer to a friend of mine. I am thinking of putting some new operating system on it is this possible? Further, I have some sensitive data on the hard drive, does any one know for definite that one if its possible to put anew operating system on my old computer, and two my friend or anyone else will be able to retrieve this data? Thanks for your time:wave