Armies of hijacked computers are flooding the world with spam as hackers devise slicker ways to take over unwitting people's machines, according to a Cisco report released Monday.

Virus-infected computers are woven into "botnets" used to attack more machines and to send specious sales pitches to email addresses in low-cost quests to bilk readers out of cash.

"Every year we see threats evolve as criminals discover new ways to exploit people, networks and the Internet," said Cisco chief security researcher Patrick Peterson.

Junk email referred to as spam accounts for nearly 200 billion messages daily, approximately 90 percent of email worldwide, according to a Cisco Annual Security Report.

The United States is the biggest source of spam, accounting for 17.2 percent of the messages. Turkey and Russia ranked second and third, accounting for 9.2 percent and 8 percent of spam respectively, according to Cisco.

This year, botnets were used to inject an array of legitimate Websites with an IFrames malicious code that reroutes visitors to websites that download computer viruses into their machines, according to Cisco.

"The botnet is, in many cases, ground-zero for online criminal threats," Peterson said.

"Using malware to infect someone's computers is an incredibly common mechanism and harnessing them all together is a way they do their click fraud, spam emails, and data stealing."

As computer security vendors such as Cisco get better at protecting machines from hackers and users grow wary of clicking on unsolicited Web links or email attachments, online criminals are turning botnets on Web-based email accounts.

Hackers are "reputation hijacking" by using botnets to figure out weak passwords protecting Web-based email accounts, according to Peterson.

Weak passwords consist of family names, birthdays, home addresses, or other terms considered relatively easy to deduce.

Once access is gained to legitimate email accounts, a plethora of spam messages sent are sent in the owners' names.