Firtly make sure the motherboard is compatible. Check if its a ATX, MicroATX or even AT (if its REALLY old). A ATX motherboard will not fit into an microATX case. But a microATX motherboard will fit into a ATX case. Now, get a screwdriver and a spot to put all your screws into. Ground yourself by touching the side of the case and unplug everything from the motherboard -IDE, SATA, Power Connecters. Then take out all PCI and AGP cards. Then unscrew all the screws and lift the motherboard out. Put the motherboard into the new case, making sure the little spacers are in the right spot. Screw it back in and plug in all the cables again.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.