Drive cables : Ensure that you have all cables for connecting the hard drive, floppy drive, and CD-ROM to the I/O on the motherboard or I/O card. These cables usually are supplied with the motherboard or drive itself, but not always, and sometimes not in the quantity you need. Another common mistake is to buy cables that are too short. A modern alternative is to get rounded data cables. They help tidy up the inside of your case as they at least appear more compact and allow cleaner air flow than would a case crammed with a bunch of wide, gray ribbon cables that often get in the way.

Audio Cable : Usually supplied with the CD-ROM(if it isn't jsut how much did you spend on it??), it connects your CD-ROM to your sound card directly.

Screws : a simple mistake and easy to overlook. Makes sure you have enough screws and that the screws are the right size. There are different sizes used for connecting card than for connecting drives, and if you try using a large screw on the drive, you'll crack the drive.

System Disk: Make sure you have a system disk setup and ready to use. You can make one for whatever operating system you plan on using. If you have another machine already running, use it to make a system disk.