How to Stop Operating-System Attacks Learn which antivirus pop-up ads may be fraudulent. Also, Mac users need security updates, too.

To use a computer safely in the 21st century, you have to be prepared to combat a growing number of constantly evolving security threats. We tell you how to beat back the top 17 risks you may not be aware of.

Your PC's operating system is under constant threat of invasion, whether the attacker's aim is to spread malicious malware or to convince you to buy protective software that is really just a rip-off. Here's how to stand guard.

Fake Anti-Malware Offers
Why You Should Care: Bogus security apps will take your money but won't clean your PC.

Scenario: Fraudulently advertised, ineffective antimalware ranks among the fastest-growing types of online scams. Products with names like DriveCleaner, WinFixer, Antivirus XP, and Antivirus 2009 are touted through online ads that simulate Windows alert messages, warning you that your computer is infected with some sort of malware and advising you to buy a particular antivirus product to fix it. Some purveyors of sham utilities embed warning messages directly into the Windows desktop, pop up messages from a System Tray applet, and install a program that generates a realistic-looking blue screen of death crash to convince you the problem is serious.

But these scareware tools only pretend to scan your computer for malware, detecting either innocuous, commonly used Registry keys or nonexistent (or planted) alien files. Even worse, many of these programs disable key components of Windows--such as the Registry editor or Task Manager--or deactivate options within Windows' Display Properties settings to prevent you from killing the programs or removing the alert messages. People are especially susceptible to these snake-oil packages because the debased sellers charge a seemingly reasonable fee (often $40 a pop) for them.

Fix: A legitimate malware remover--one that independent testing has objectively demonstrated to be effective--should be able to deal with the immediate problem of an adware program that won't let you remove it. Check your security software to see if it will do the trick. But the real fix may be concerted government action: Late last year the Federal Trade Commission asked a federal court to stop some perpetrators of this type of scam. It may be that prison terms or massive fines are the only useful deterrents.

Zero-Day Attacks
Why You Should Care: A PC is most vulnerable to attacks launched before the software maker has devised and released the necessary fix.

Scenario: For a number of years, Microsoft has scheduled the release of most of its security patches for the first Tuesday of each month. But in the final quarter of 2008, two security patches for Internet Explorer--known as MS08-067 and MS08-078--were so urgent that the company released them immediately, without waiting until the next "patch Tuesday" Rolled around. Such off-schedule releases are known as "out-of-band" patches. The release date for each was rushed forward when experts detected attacks in the wild that exploited vulnerabilities corrected by the patches.

Microsoft's need to release an occasional out-of-band patch is perhaps inevitable, and the company delivered the two IE patches with commendable speed. But the occurrence of two high-profile, out-of-band releases within two months might signal a worrisome trend.

Fix: Obviously, Windows' Automatic Updates will eventually install the patches you need. But the Automatic Updates tend to roll out slowly, leaving your PC vulnerable during the critical time between the public release of the patch and the moment when you install it.

There is no technical fix for this danger. You just have to keep up on the latest security news and visit update.microsoft.com as soon as you hear about any out-of-band patches, rather than waiting for Automatic Updates to kick in.

Malware for Mac Users
Why You Should Care: Overconfidence may breed lax security practices among Apple adherents.

Scenario: Do criminals avoid targeting the Mac OS because the operating system's security profile is superior to Windows', or is it simply a numbers game? Advertising that touts the Mac's supposed invulnerability to invasive attacks has encouraged arrogant obliviousness among Apple users toward their beloved OS's shortcomings. In fact, Macs are subject many kinds of security problems, including malware that employs deceptive techniques to fool users into installing it. (For instance, Apple issued new QuickTime and iWork 09 security warnings in late January.)

Not only has the Mac OS proved to be riddled with dozens of security vulnerabilities (as evidenced by 61 security-related patches last year), but it has been targeted by DNSChanger malware (also widespread on the Windows side of the fence), which modifies a computer's DNS server settings. If bad guys can control where your computer resolves domain names, they can steer your browser to any server of their choosing, which gives them a big advantage in promoting phishing schemes and which may enable them to replace legitimate Web advertisements with ones that they stand to make a commission from.

Fix: If you use a Mac, don't assume that your system is impregnable. You need to keep up with security updates just as Windows users do--both the automatic updates from Apple and patches for third-party software (such as Adobe Reader, Flash, Java, and Office) whose makers may not automatically alert you that a new version is available. If your Mac contracts either the DNSCharger Trojan horse or the iWorkServices Trojan horse you can download a removal tool from SecureMac for either one.