Hello Chex, and welcome to Sevenforums!
Let me see if I am able to help you out with these problems you are having. From what I can tell, you are able to see part of your desktop, but it seems like it is just centered so that the toolbar and start menu are gone. If you move your cursor to any side beyond the monitor's screeen, it disappears from view into nothing until you pull it back.
Can you Right-click the desktop to where a list shows up, and then select "Screen resolution"? Will the window pop up? You might try lowering the resolution before you connect the HDMI cable to 1280x720 (lower if needed), and then adjust higher later on. Let me know how this works for you. If it doesn't, please let me know the make and model of your computer and the type of graphics card you have (If it is integrated, let me know). If you need further help or would like me to explain in more detail, don't hesitate to ask.