Has your computer been acting a little funny lately? If so, maybe you've loaded it down with too many photos or music files, or it needs a new battery or hard drive.
Another possibility: Your PC has been hijacked by a shadowy cybercrime syndicate.
If so, you're not alone. In recent years, hundreds of millions of desktop and laptop PCs have been taken over by hackers -- generally without the owner's knowledge -- and pressed into round-the-clock digital slavery, sending spam, fishing for credit card numbers, spying on Internet traffic or logging users' keystrokes. They have become part of one of the Internet's largest security problems: the botnet.
A botnet is an army of infected computers -- bots -- all remotely controlled by hackers. Huge numbers of compromised PCs can be very powerful when linked together, and hackers can direct their botnets to perform unsavory but often lucrative tasks. Computers get infected when the user unwittingly opens an infected file or email attachment, or visits a booby-trapped website.
Better relations between law enforcement and the professional security community have led to the dismantling of some of the largest botnets, including one this year that had been capable of sending an estimated 30 billion spam emails per day.
Still, security experts say dozens of huge botnets remain, and the flood of valuable personal and financial data online has made botnet hacking a big business that will be difficult to eradicate.
"The good guys are ahead on some fronts, but the bad guys have some very capable people," said Don Jackson of SecureWorks, a Dell Inc.-owned security firm. "It's always going to be a chess board with just two kings on it."
So what can you do to keep your computer safe from the bad guys?
Just as with physical health, preventive care helps you avoid nasty problems rather than go through the stress of trying to treat them.
Watching what you put into your computer is paramount: Avoid downloading programs from the Internet, opening strange email attachments or visiting unfamiliar websites. Those are the most common modes of infection.
Next, think of your computer's anti-malware programs as regular exercise. Run them frequently, and keep them up to date. Viruses and nasty programs have a much easier time infecting unprotected computers, or those with expired antivirus software. As with certain illnesses, once your computer is infected by one bug, it becomes more vulnerable to repeated attacks.
If you're looking for security software, try well-known commercial offerings like Norton AntiVirus, McAfee AntiVirus Plus or any of a number of reputable free products like AVG or Avast, both available online.
Most newer versions of Microsoft Windows have security features built in, but users should allow the feature to download updates whenever needed. Users running the decade-old Windows XP (and, according to Microsoft, there are still 200 million people who do) can download and run Microsoft Security Essentials, the company's free antivirus program.
If your computer is already infected and has been turned into a bot -- also called a "zombie" -- bringing it back to the good side can be tricky. The malicious programs that take over your computer are designed to operate without being detected by security programs, and can often root themselves so deeply in a computer's system that removal can be a technical challenge for most users.
If your computer is performing slowly, popping up strange ads or messages, or is otherwise behaving oddly, try running one or two of the antivirus programs mentioned above. But make sure they are fully up to date with the most recent virus "definitions" (so the computer knows which bug to find). The security software may be able to fix the problem before it progresses too far.
If things continue to look grim, do not despair. You still have options, though they may be more time-consuming or expensive.
First, if you want someone else to handle the problem, you can take your computer to a security specialist. Try using Yelp to find someone locally who has been well reviewed by at least a dozen customers, and poorly reviewed by few. Professional help could run you $100 to $200, but may save you peace of mind.
For readers interested in a more do-it-yourself approach, the Web offers several communities of online security helpers who can assist you in diagnosing and repairing the problem -- free of charge. On sites like Techguy.org, users can get help by posting diagnostic information that enables helpers to pinpoint problems and offer solutions. This process can be very technical and require days of back and forth, but can often be fruitful and informative.
Once you get your problem fixed, remember to remain vigilant. It takes only one bad click to convert your mild-mannered computer into a zombie spambot.