What's the most vulnerable computer part? The user?

Little by little, the loop is closing on some of the biggest threats to personal computing.

Viruses, spam, hackers and spy ware -- all are technological problems with technological solutions.

Already, programmers are building ever tougher firewalls, ever smarter spam filters, ever better anti-virus software. In time -- and it won't take more than a couple of years -- our computers will be far less vulnerable to outside attack.

But what about the humans?

Over and over again, we computer users have proved to be suckers for bogus come-ons. It's no exaggeration to say that some people will double-click anything you send them. That's a vulnerability far tougher to patch than a mere security hole in your browser.

Last week we saw a case in point: the so-called "MyDoom" virus that swept across the Internet, crippling e-mail systems and attacking Web sites.

Although popularly described as a virus, "MyDoom" was actually a much more insidious threat. It actually combined the qualities of a virus with that of a "Trojan Horse" -- a program that, as the name implies, looks innocent but isn't.

The distinction between a virus and Trojan Horse may seem like an obscure technological nuance. But if you look more closely, you'll see a menace worse than any virus the Internet has yet delivered to your in-box.

Trojan Horses masquerade as something fun or useful, like a photo or an electronic birthday greeting. But when the recipient clicks on the program, which usually arrives as a file attachment, the malicious code springs to life.

From there, the Trojan Horse program can make your computer do pretty much whatever it wants.

Send your credit card numbers and passwords to some hacker? You bet. Erase your hard drive? In a heartbeat. Copy itself by e-mail to all your friends? Sure thing. Open your PC to remote control by others? No problem.

"MyDoom," however, exploited the vulnerability of the computer users, not the computer. It merely waited for users to click on it. And click they did, if worldwide reports of e-mail and Web disruption aimed at The SCO Group (a software company) and Microsoft are any indication.

This, of course, was no surprise to the bozos who write these malicious programs. They know that users are far easier to fool than virus scanners.

But, short of banning file attachments, how are you ever going to stop people from impulsive clicking? Even if you convince 99 percent of users to be scrupulously careful, the remaining 1 percent is more than enough to make the whole effort worthless.

Savvy software and hardware specialists will soon cement the holes remaining in our computers. But their job won't be finished until they also figure a way to save us from ourselves.