I know HDMI > DVI adapters exist, but I have still yet to see a DVI> HDMI. I can see why they wouldn't bother making one and i'm not even sure if you would be able to do it with HDCP.
what are you trying to do? i think i understand but you have the input and out put backwards. it looks like you mean to have the video from the xbox and pc to go to a single monitor. if so then you would be better getting a switch box.
You need a DVI or HDMI switcher. Since your computer and your display have DVI, I would use a DVI switcher because it will require fewer adapters. Here is an example of what you'll need: