Take the precautions you would with any piece of valuable equipment to protect against casual theft. This step can include locking the room the computer is in when no one is there to keep an eye on it, or attaching the unit to a wall with a locked cable. You might also want to establish procedures for moving or repairing the computer, so that the computer or its components cannot be taken under false pretenses.
Use a surge protector or power conditioner to protect the computer and its peripherals from power spikes. Also, perform regular disk scans and defragmentation to isolate bad sectors and to maintain the highest possible disk performance.