Before you use the canned air or Air Compressor on the inside, make sure to remove those pesky SATA Cords as they tend to grab and hold onto pet hair and other dust chunks. Be careful when reassembling as the ports are numbered. 0 is the first SATA port your motherboard will load so make sure your HDD is plugged into 0.

Your Optical Drives plugs into the number 1 which is right next to your 0 port. 1 will be loaded second so make sure your Optical Drive is connected to that. Otherwise when you turn your PC on, it will not load your Operating System, which is the Interface you see when using your PC.