certified means youre getting something that will actually work, not certified.. you run the risk of buying some very cheaply made cable.
as for your question.. hdmi for your xbox? im not understanding.. hdmi computer monitor? xbox 360 or regular xbox? What does the connection look like in the back?
these are the 3 most likely connections..
the lastone is a VGA which is what you could have in the back of the computer.. this is an analog signal so its not compatible with hdmi, youd need a scaler which would cost a couple hundred.. not worth it.
but you can get a vga to dvi. or dvi to hdmi.