As some humorist once quipped, "Just because you're paranoid doesn't mean that no one is out to get you." These days, when real and imagined mischief makers seem to be everywhere, a little healthy paranoia about safeguarding our computer systems and software is undoubtedly a desirable thing.

While hackers have been plaguing government and business computers for decades, the stakes have risen dramatically in recent years as security agencies warn of the distinct possibility of cyberterrorists striking at the nation's computer infrastructure. But in the Pavlovian world of computer programming, every stimulus creates an almost instant response. As a result, companies around the world have been working tirelessly to assure that for every area of vulnerability, preventative measures, detection techniques and countermeasures are available. Many of these are being provided to computer users at no cost.

Dozens of these free programs and services can be downloaded from numerous shareware, freeware and software publisher sites including C/Net (www.download.com); Tucows (www.tucows.com); and Z-D Net (www.zdnet.com) as well as many others.

The following services are a sampling of some of the security programs available at no charge:

• The Center for Internet Security (http://www.cisecurity.org) offers free security benchmark and scoring tools that can be downloaded for the most common operating systems. These allow you to determine security weaknesses in your system and suggest remedies.
• SOFTWIN (http://www.bitdefender.com/), based in Romania, has free tools for removing many of the most common viruses that exploit security loopholes in Instant Messaging Software (IMS) applications.
• Gibson Research Corp. (http://grc.com/) provides online Internet security checkups and information. Among other things, Gibson's site can analyze your firewall to determine whether it can be fooled by "Trojan" programs or viruses. Gibson also has a free program, "UnPlug 'n Play", which the company says "easily disables the dangerous, and almost always unnecessary Universal Plug and Play service."
• McAfee's SecurityCenter (http://download. mcafee.com) offers free security services, including a Security Index and real-time external security alert system which allow you to determine your PC's security vulnerability.
• Zone Labs (http://www.zonelabs.com/) allows you to download its bare-bones "Zone Alarm" firewall, considered by many to be one of the best firewalls on the market, free of charge at the company's Web site.
• Avast (http://www.avast.com) offers a free antivirus program that lets you scan your hard drive for viruses. The company Web site also has free time-limited demos of its other anti-virus programs.
• Trend Micro (http://housecall.antivirus.com), the publisher of PC-Cillin antivirus software, offers a free online virus scan called "House Call." While Trend requires that you register to obtain the service, using House Call has the advantage of always employing the most current virus database.
• Citi-Software Ltd. (http://www.citi-software.co. uk), based in London, is giving away a personal password manager with built-in, high-level encryption security called "Access Manager." The program allows the user to record and have immediate access to unlimited encrypted PINs, passwords and other confidential information.

But you don't need a password or a PIN to send your comments and/or suggestions to harper.d@att.net.