We're all aware of the havoc that viruses can wreak on our computers. We buy antivirus software. Still, that doesn't guarantee that our computer won't be hijacked by hackers. Such software can help, but there's always a risk. Hackers are always inventing new ways to compromise our computers.

The clean-up costs due to damage can be extremely expensive, and millions of dollars can be lost in terms of economic productivity. Of course, some viruses and worms are vastly more expensive than others. Here are the 12 costliest computer viruses and worms we've found:

12. Morris: $10 million

Recognized as the first widespread worm/virus on the Internet, the Morris Worm crippled the burgeoning Internet in 1988. Using MIT's systems to hide the fact that he was a Cornell graduate student, Robert Morris released his creation onto the web and infected more than 6,000 computers. Even though Morris meant it to be harmless prank, it turned into a major threat. The U.S. General Accounting Office estimates that damages may have cost as much as $10 million. Some individual computers cost more than $53,000 to fix. Ten million isn't much compared to the other viruses on this list, but the Morris worm still deserves a spot for its historical contribution as the world's first massively expensive computer infection.

11. Blaster: $320 million

In 2003, the Blaster worm, also known as LoveSAN or MSBlast, unleashed itself on Microsoft operating systems. The scope and cost of the infection was huge, with hundreds of thousands of computers affected. One of the messages left behind by this fast-spreading worm included an admonishment to Bill Gates, asking him to stop making money and fix his software.

10. Sasser: $500 million

In April of 2004, the Sasser worm was unleashed by a teenager in Germany, targeting Windows XP and Windows 2000. The worm actually crashed the coastguard mapping system in Britain and resulted in Delta Airlines canceling flights. The French news agency AFP also had all of its satellite communications blocked for several hours. The creator of the worm was caught quickly. One of his "friends" turned him in to collect a $250,000 bounty posted by Microsoft.

9. Nimda: $635 million

The Nimda virus ("admin" spelled backward) made an appearance in 2001. It showed up via several different vectors - email, infected Web sites and attacks on Microsoft IIS servers. Within 22 minutes of its release, it became the Internet's single most widespread virus, allowing it to cause a great deal of damage.

8. SQL Slammer: $750 million

Another famous worm from 2003, the SQL Slammer downed banks and slowed Internet access globally. Estimates of cost ranged between $750 million and $1.2 billion. As many as 200,000 computers were affected. Pinpointing exact costs are difficult, of course, since it's hard to put a price on lost productivity and because these worms can continue to quietly propagate for years.

7. SirCam: $1 billion

In 2001, many businesses were pounded by the SirCam virus, which included a file that, when opened, instigated an attack. Fortunately, the spread of SirCam was limited because many people updated their Internet security. SirCam still did plenty of damage. However, it was considered a "flop" that didn't live up to some of the more spectacular viruses that came before.

6. Melissa: $1.2 billion

Named after a stripper, Melissa was extremely sleazy, and was spread from alt.sex, a Usenet discussion group. This virus, famous in 1999, didn't do a great deal of overt damage, but it still clogged up many networks and caused widespread problems.

5. Code Red: $2 billion

Code Red is one of the most famous viruses because it simultaneously spread a number of worms over the Internet. Different versions of Code Red continued to crop up throughout 2001. The virus had an ability to scan the computers it infected and infiltrate their networks. Code Red caused a great deal of damage, and Code Red II was meant to allow hackers to control infected computers.

4. Conficker: $9.1 billion

The Conficker worm was released in 2007, infecting millions of computers around the world. Once a computer was infected, this worm downloaded and installed malware from sites controlled by the hackers, including things like keystroke loggers and remote PC-control software.

3. ILOVEYOU: $15 billion

This virus was first noticed in early May of 2000, and within a week it already racked up damages to $5.5 billion. It was unleashed via an email that contained a simple and endearing subject line - "I love you." However, once the "love letter" attachment was opened, the virus went to work on the system, replicating itself and sending itself to everyone in the email's contact list. As the virus spread, so did the cost, ratcheting up ever higher. And ILOVEYOU had another effect: It popularized the use of email attachments by virus and worm creators.

2. SoBig: $37.1 billion

One of the reasons that some worms and viruses become extremely expensive is because they have the ability to spread, and versions of them continually circulate on the web. The SoBig virus is one of these. Started in 2003, this virus quickly spread, acting as email spam as well as a virus. The widespread effects of SoBig continue, and replication means that there is a truly global reach for this virus, which emails itself via attachments.

1. MyDoom: $38.5 billion

MyDoom takes the cake as the worst malicious code in history – at least up to this point. In 2004, MyDoom took the Internet by storm, causing all sorts of damage and forcing the mi2g consultant group to estimate damage at more than $38 billion. In addition to being so widespread, some believe that MyDoom is also the fastest spreading virus ever. It only took a very few days for MyDoom to be almost everywhere, propagating at an increasing rate. At one point, 25 percent of emails in circulation had been infected by MyDoom.