Now that the case is open, now is a good time to go through the screw supply provided with the case. These are usually held in a small plastic bag nestled inside the case. Inside this bag you should find: 

Chassis screws - this is the type used to tighten down cards, etc. 

Smaller screws - just like the chassis screws, just with a smaller diameter. It is used to fasten the motherboard in. 

Standoffs - these are screws that are used to hold the motherboard about 1/8" from the motherboard mounting plate. Their ends have a threaded opening in them that accept the smaller chassis screws. If you have an AT case, you may find small white standoffs. These serve the same function as the metal standoff, but are simply punched through the board and slid into slots on the case. They are rather clumsy to use compared to the metal standoffs, but they get the job done. 

Washers 

Clean Case. If the case is new, this should be no big deal. But, if the case has been used before, it could probably stand a cleaning. Clean out the inside with a rag and compressed air. Make sure the fan in the power supply is free of furry dust. Also take a rag and wipe it off. 

Inspect the Power Supply. Make sure it is tightly attached to the case, make sure it is free of dust, and make sure it is set to the proper voltage of your area- 110V for U.S. and 220V (I think) for outside countries. 

Inspect Power Switch. Make sure the power switch is securely tightened and correctly connected to the power supply. With most AT cases, the power switch is already connected to the power supply by four wires. In ATX cases, the power switch will have one loose wire coming off of it. This wire will then connect to the Power Switch connector on the motherboard. The power supply should be attached to the power switch already and the connectors should be covered with electrical tape. 

Install Feet. These are little tabs inserted into holes at the bottom of the case. The case sits on these tabs when on your desk. If the case has been used before or it is a more expensive case, this may not need to be done. 

Install Case Fan. Sometimes, you may want to install a separate fan that screws onto a rack next to the vent on the front of the case. This helps increase circulation of air through the system. Make sure the fan is set to draw air into the case, not blow out. Many cases already have this installed, so you may not need to worry about it. Some like to put a little filter over the hole so as to prevent dust from be drawn in. 

Configure the LED. The LED on the case operates completely separate from the actual speed of the system, so you can set that now. It is done with jumpers on the back of the LED. You will need the little manual that came with the case to do this right. Many newer cases don't even have LED's, so don't worry about it.