Unknown Entry - S-1-5-21-2533013469-2641548184-613786714-1000

I have recently discovered the entry
S-1-5-21-2533013469-2641548184-613786714-1000 in the security setting
section of the Properties box when i right click on an object, be it a
program or a shortcut.

This entry show a person with a qiestion mark next to it, I do not remember
adding it and it appears to have special permission attached to it. I have
tried to get rid of it however some items such as the boot area refuse to
let go of it.

1. What is this ?
2. how to get rid of it competely ?

Thankyou in advance