The number of Windows computers that have been commandeered to send out spam e-mail increased by a quarter last week because of a new virus, a security company said Tuesday.

The new virus, whichemerged Aug. 12, is an updated version of Mocbot,first released to the internet in 2004.

It targets a security flaw in Windows computers that Microsoft fixed with an update released Aug. 8. The virus appears to be targeting only computers running Windows 2000.

Mocbot creates so-called zombie PCs, a network of compromised computers that are controlled remotely by virus writers. Usually, zombie computers are used to send spam e-mails or to spread more viruses.

Security firm CipherTrust said Tuesday that the number of zombie PCs they detected increased to 265,000 in the past week from 214,000.The increase was a result of the new Mocbot virus, the company said.

The company counts zombie computers by comparing e-mail spam originating from known zombies to spam coming from previously unknown sources. In the case of the new Mocbot virus, the spam is mostly for fake Rolex watches and pornography, CipherTrust said.

Spam makes up 81 per cent of e-mail traffic, the Georgia-based company said.

CipherTrust predicted the number of zombie computers would return to a more normal level once people install the security patch released by Microsoft.