Testing a recent variant of Zeus on the VirusTotal Web site, Warner found that only five of the 41 antivirus detection systems used by VirusTotal managed to spot it.

Although antivirus vendors have other techniques for blocking the malware -- they can stop people from visiting the malicious Web sites, for example -- the spam is giving the companies a run for their money.

"It's difficult to stay ahead of it via antivirus because the Zeus binaries are changing a few times a day to evade detection," said Paul Ferguson, a researcher with , via instant message. "It's definitely a problem."