Some details about denial-of-service attacks, like the one Thursday against Twitter. To picture a "denial-of-service" attack, think about what would happen if you and all your friends called the same restaurant over and over and ordered things you didn't even really want. You'd jam the phone lines and overwhelm the kitchen to the point that it couldn't take any more new orders.

That's what happens to Web sites when criminals and hackers hit them with denial-of-service attacks. They're knocked offline by too many junk requests from computers controlled by the attackers. The bad guys' main weapons in such an attack are "botnets," or networks of "zombie" personal computers they've infected with a virus. The virus lets the criminals remotely control innocent people's machines, which are programmed to contact certain Web sites over and over until that overwhelms the servers that host the sites. The servers become too busy to respond to anything, and the Web site slows or stops working altogether.

People try denial-of-service attacks all the time, but assaults are often unsuccessful because Web sites have ways of identifying and intercepting malicious traffic.

Sites that go down generally are less prepared, because they are less accustomed to being hit or aren't sensitive enough to warrant extra precautions.

Popular Web sites, like e-commerce and banking sites, have a lot of experience dealing with denial-of-service attacks, and they have sophisticated software designed to identify malicious traffic.

If your computer is being used in a denial-of-service attack, you're likely to see a significant slowdown, because your processing power is being siphoned for the assault. But there aren't always obvious signs that your computer has been infected.

So the best thing is to focus on prevention, namely by having up-to-date antivirus software. If you're concerned your machine might be infected, it's wise to run an antivirus scan. Many antivirus companies offer a free scan from their Web sites.