So, what are the really dangerous viruses? I'm talking about viruses that do more than erase your hard drive--they make your hard drive unusable. Two such viruses are predicted to strike later this month.

First, let me answer a basic question: Can viruses damage hardware? Technically, no. But viruses can overwrite or corrupt your computer's CMOS and BIOS so that it will not boot.

Firmware is the read-only memory that tells the computer what to do before the operating system loads. The CMOS stores information regarding the date, time, what peripherals a machine has, and where to find them. The BIOS allows a computer to communicate with the keyboard, display, and drives before the operating system is loaded. The viruses I'm talking about today render this important information, and therefore your computer, unusable.

One of the earliest, and most famous, virus to damage a PC's BIOS is Chernobyl, or CIH. The CIH virus, triggered on the anniversary date of the Chernobyl nuclear disaster, April 26, infected more than a million PCs in Korea and resulted in more than $250 million in damages. Be aware. CIH is still around, and its second anniversary is in two weeks.

Similar to CIH is another virus called Kriz. Triggered on December 25, Kriz clears the information in the BIOS. If you have the Kriz virus, Symantec has a tool to remove it. Even if you don't have Kriz, its threat still lingers. This past December, several antivirus companies noticed that Kriz had piggybacked with other, newer viruses such as Bymer. That's yet another reason to keep your antivirus signature files up to date.

Now there's a more sophisticated virus called Magistr. Because it is a mass mailer, like Melissa, Magistr can spread quickly. Since it changes its subject, body, and attached file names with each new infection, Magistr can also be tricky for antivirus software to detect. What started as a trickle of reports worldwide has become a steady stream. Within the last month and half, Magistr has climbed from obscurity to the penultimate position on the MessageLabs top threats list.

Last Friday, April 6, Steven Sundermeier of Central Command told me that based on the high number of inquiries received in recent days, his company went ahead and developed a new Magistr removal tool. Another antivirus software company, Kaspersky Anti-Virus, warned on Monday, April 9, that the number of Magistr reports we're seeing right now is still low only because of the one-month delay between its initial infection and its destructive trigger date. They predict there will be a noticeable increase after mid-April.

Whether or not your PC gets CIH or Magistr, it's best to be forewarned. If you haven't updated your antivirus software and scanned your system, do so now. You'll save yourself a lot of grief and avoid having to take your PC into the shop.