Fortunately, most power supplies live longer than the PCs they power. But if your supply does fail, you can pick up a new one at a good computer store for around $50.
The hardest part about replacing your power supply is finding one that fits. Many PCs use power supplies that conform to a common motherboard form factor such as ATX, AT, or Baby AT. Look at your system manual or check with the vendor, and be aware that certain suppliers sometimes tweak the form factor.
Make sure your new power supply has enough connectors for attaching to hard disks, floppy and CD-ROM drives, and other components. Your best bet is to remove the old power supply and carefully compare it to any new supply before making a purchase.
Swapping power supplies is easy. First, unplug the computer. Make a detailed drawing of which connectors go where -- especially on the motherboard -- and disconnect. Undo the four screws on the outside of the case that secure the power supply. Gently lift it out. To install the new supply, reverse the process.

Grabbing extra power

Most PCs sold today come with at least a 200-watt power supply -- plenty of capacity for an average configuration. But the supplies in many older systems, and especially in low-cost clones, may not be powerful enough to support additional demands. So before adding any extra component to your system, make sure your power supply can handle it.
First, find your power supply's maximum wattage capacity. It should be written on the outside of the supply case or in your system documentation.
Compare this number to your PC's power needs by adding up the individual wattage requirements of each component (including any new devices you wish to add). Look under the technical specifications in your PC's manual or in the documentation for the peripheral. If you can't find the wattage numbers for a component, look for the voltage and amperage, then calculate the power usage in watts by multiplying volts by amps.
You can also call or go to the Web site of your PC's manufacturer -- or better yet, the maker of the component -- to get the wattage numbers. If even that doesn't help, use the chart below as a guide.

Surge and rescue

Sudden spikes or surges in the AC voltage caused by lightning can literally burn up your PC's delicate circuitry. To protect your system, you need a surge protector. A good one will cost $80 or more. Make sure it has an indicator to warn of exposure to excessive voltages. Most metal oxide varistors -- the mainstream surge-protector type -- can lose their ability to protect after absorbing a single high-voltage surge. An indicator will tell you that the device is suspect and should probably be changed.
Even if you are protected from a surge, you still may lose vital files if the surge is followed by a power outage or blackout. To keep your PC running for a few more minutes so you can shut it down properly, get an uninterruptible power supply -- basically just a large package of batteries. The $240 APC Back-UPS is a good "entry-level" UPS, while more expensive units -- such as the $340 APC Back-UPS Pro -- provide extra functionality, including the ability to shutdown applications automatically (APC, 1800 652 725).

Beef up your notebook battery

Nowhere is power management as critical as in notebooks. Here's a rundown on current notebook battery types:
Nickel cadmium. NiCds are very common in older notebooks. Most but not all NiCds suffer from a "memory effect" that can reduce their storage capacity when they are only partially discharged, not used for a short time, and eventually recharged. The cure? Completely discharge the battery and then immediately recharge it. This should be done with the battery installed in the notebook.
Nickel-metal hydride. NiMH batteries provide slightly better performance than NiCds, lack the memory problems, and are common in newer budget notebooks.
Lithium ion. The notebook battery type of choice, lithium ion outperforms NiMH, is no heavier, and is found in most newer power notebooks.
No battery, of course, lasts as long as you'd like. Notebooks offer a bewildering choice of power-saving modes that may be activated by keystrokes, through the system setup program, or -- if the notebook supports Advanced Power Management -- through software.