No matter what else you do, regularly back up your critical data. Good back up software makes the process easy, efficient and more likely to get done. Pick a reputable product, familiarize yourself with its features and procedures, decide on your strategy-and do it.

When you're backing up data files, keep at least three back up generations, rotating among them. This can save you if your system crashes during a back up, which renders both the primary and back up data unusable. Back up on different cycles; if you don't catch a problem (or virus) until it gets propagated into recent back ups, having older copies to fall back on may save you. To protect from a major disaster such as a fire, at least one generation should be kept off-site, but in the regular rotation. Finally, test your back ups occasionally and retire suspect or aging media; they may let you down when you need them.

Keep at least one back up of your system and other software as you have it configured, and update it whenever you make significant changes.

For critical applications requiring continuous availability, consider a redundant array of independent disks (RAID) system, along with other fault-tolerant hardware. In its back up incarnation, a RAID system mirrors data simultaneously to two drives. This means that one can fail and be replaced without any loss of data. RAID must be supplemented with off-line back ups, however, since it doesn't protect against the propagation of errors.

Due to the vagaries of the power grid, your computer experiences power "events" every day. These "spikes" and "surges" range widely in severity and duration. Computers are designed to deal with most of them, but additional protection is an excellent idea, particularly if your area is prone to power problems. Severe surges may overwhelm your computer's defenses, and smaller ones can cause cumulative damage. The surge-protecting power strip is the most commonly used defense against "dirty" power, but don't expect that $7.95 strip to perform as well as the $44.95 one. Also, some surge-protection components are "sacrificial," so look for one with an indicator that tells you when it has given up the ghost. Though the voltages and currents are lower, telephone and network wiring is subject to events similar to those on power lines. Surge protectors are available, and telephone line protection is often integrated into power strips.

If your area has a history of power failures, you may need an uninterruptible power supply (UPS), which will run things on battery power for a limited time. Starting at around $80, most units have effective spike/surge protection built in. (Incidentally, one advantage of a laptop besides its portability is that it has built-in UPS.) If you're providing multiple layers of protection, don't plug a surge-protected power strip into a UPS unless the operating manual specifically allows this, and remember to put it upstream of the UPS. Also-it should go without saying-but never plug a UPS into itself.

Upscale UPS's offer more features and the ability to power larger loads for a longer time. If your area is subject to brownouts, invest in a UPS that can raise input voltage to normal without using battery power; a unit without this capability will quickly exhaust its battery trying to maintain proper output. If you operate an unattended server, look for a unit that supports software for your platform that can automatically perform a graceful shutdown of the connected computer(s) before battery power runs out. Some UPS's can even notify you by pager or other means if they experience a power event.

Computers are protected against static electricity, but static can be the real culprit in behavior often attributed to other causes. Your best line of defense against static is adequate humidity, particularly in the winter. Also a good protective device is a static strip that is connected to a ground. Look for one, such as a keyboard stick- on, that you'll likely touch first automatically. Static electricity can go through your body and into your CPU unless you discharge yourself with one of these strips.

While hard drives have grown increasingly reliable, they remain subject to many potential problems. However, there are good drive utilities (such as the popular Norton Utilities) for both prevention of and recovery from many maladies. Install one of these before you have problems. Read the manual that came with your utility and follow its recommendations. Regularly perform a thorough scan of each of your drives (including removables such as Zip and Jaz disks). Scans can detect bad blocks on the media and preclude data being written to them. They can also find (and often fix) developing corruption that otherwise might later render a file unusable.

In order to make efficient use of drive space, a modern operating system will break files into pieces and scatter them around the drive if it can't find room to store them in contiguous blocks. This fragmentation both slows down file access and increases wear and tear on the drive. A drive optimizer will put these fragments back together and slow further fragmentation.

Before optimizing, always back up your data. The optimizing utility shuffles a lot of stuff around the disk, and if something should interrupt this process (such as forgetting to plug in your laptop and having the battery poop out), you may wind up with a badly hosed disk. Never let your free space drop below 10 percent of the drive's capacity, which will accelerate fragmentation and preclude optimization.

If you experience a file loss or other drive problem, take a deep breath and don't panic. Do not attempt to save, move or erase any files on your own-you may well walk on data you want to recover. Before doing anything else, read your utility's instructions for your situation. If you attempt data recovery, carefully read onscreen messages and heed suggestions. If you suspect a hardware problem, power down the drive immediately and call an expert.

Computer viruses are an unfortunate fact of life these days, particularly if you go looking for software online. If you never connect your computer to the outside world and never add new software, you have nothing to fear. But computing in such a sterile environment is not practical.

There are several good anti-virus programs for every platform. By recognizing the telltale signs of virus code, they can alert you to the presence of a virus before it infects your entire computer. Most of these programs can also eradicate many viruses and repair damaged files. Look for an anti- virus program that has frequent updates to incorporate new virus definitions. These can be obtained on the Web or by email.

A new type of virus has reached epidemic proportions since the release of recent versions of Microsoft Word, which now includes a macro language that allows the automation of many functions. Unfortunately, it also can be used for nefarious purposes on any platform running Word, and is relatively easy to use, for good or ill intentions. Make sure that any anti-virus program you buy can deal effectively with Word macro viruses, and keep up on updates, since new Word viruses are appearing virtually daily.