I googled 'The Office IT Guy' and came up with the same YouTube clip that was described as: 'Simon, "Tim's favorite computer geek" (as Dawn says), discusses computers with Tim, and go-karts and Bruce Lee with Gareth'.
According to Imdb, the character of 'Simon' is played by actor Stephen Humby.
http://www.imdb.com/title/tt0664496/fullcredits#cast