Abstract: An uninterruptible power supply (UPS) is electrically coupled to a main board, a hard disk and a power supply of the computer. The UPS comprises a power detection circuit for detecting a power supplying state of the power supply when the computer is halting, a power management circuit for receiving a blackout signal sent from the power detection circuit when a blackout in the power supply is detected, and a power switch circuit is commanded by the power management circuit to switch power of the main board and the hard disk from the power supply to the rechargeable battery for maintaining a normal operation. At the same time, the power management circuit sends a power ready signal to the main board through a power switch on the main board to cause CPU to boot the computer normally and the CPU commands to store temporary data and programs in the memory in the hard disk. Such safely stored data and programs may be retrieved from hard disk by CPU for use when computer is powered on again.