When a desktop computer loses its horsepower or, worse, crashes entirely, it seems a shame to throw it away, and often unnecessary: even the most timid computer user can add a U.S.B. hard drive or connect a new monitor.

But when a laptop dies, it seems like it’s the end of the line. However, new online stores and easy upgrades mean that wounded laptops can be healed.

PREVENTIVE MEDICINE Doing a good cleanup is usually a good idea when trying to give an older laptop a new lease on life. Compressed air is great for blowing out stray strands of hair or dust in the keyboard and DVD drive, and rubbing alcohol will clean up most surfaces.

Now that the outside is clean, you can look at the operating system. Most systems fail or slow down because of faulty software. Windows 7, the new operating system from Microsoft arriving Thursday, will include virus and spyware removal software.

But if you have an older system, you can download Avast antivirus software (at avast.com) and AdAware (at lavasoft.com), two free programs that work to clear up unruly computers. Mac users, luckily, have little to fear from spyware and viruses.

COMMON PROCEDURES If things are still broken — or you’re not happy with the results — a fresh installation of the operating system may be in order. However, if the problem is deeper in the hardware — perhaps a balky hard drive or too little memory — there is hope.

Most laptops consist of a motherboard hidden underneath the keyboard, and thus inaccessible to mere mortals, and peripherals intended for easy access. The meaning of “easy access” varies according to manufacturer, but generally the memory and hard drive are accessible through panels on the bottom or side of the case.

A word of warning: many do-it-yourself upgrades or repairs can void warranties and, given the wide variety of laptops out there, there is no defined set of steps to follow in order to perform these improvements.

LOSS OF MEMORY, DISORIENTATION If you are dealing with a hard-drive problem, the symptoms will include start-up failure as well as slowness and “grinding” — essentially unnecessary reading and writing to the disk — as well as warnings that you’re running out of disk space. If you have a more severe problem, however, the system will fail to boot or start up completely, giving you an error about a missing operating system. This is true for both Macs and PCs.

Most laptops run faster when given a little more memory, so a general upgrade is always a good idea.

However, problems with memory are often quite nefarious and result in frequent crashes while working, resulting in freezes. Memory problems, therefore, are much harder to diagnose. Best to consult a professional.

A NOTE ABOUT INSURANCE Be sure to back up your computer before trying any fixes because no upgrade is foolproof.

You should always hope for the best but expect the worst when fooling around with hardware.

TRANSPLANT ELIGIBILITY The first step is to find your laptop model online and divine what type of memory and hard drive it takes. For Apple laptops, check out sites like ifixit.com or macsales.com. IFixIt offers step-by-step instructions for adding memory and hard drives to almost any Macintosh model. (NYT)