Security firm Webroot Software has identified five million websites hosting malicious spyware during October.

One of the most widespread infections in the UK was Comet Cursor, a piece of adware capable of tracking a user's online habits and transmitting the information to a third party.

Adware also delivers pop-up ads which can cause overall system performance issues and dramatically slow web browsing.

Webroot's monthly analysis of the most common forms of malicious attack identified Downloader-Zlob as the most common Trojan in October.

The malware has dominated Webroot's list of the most common and critical threats for over six months, silently downloading rogue security programs such as AntispyStorm, Ultimate Fixer and Ultimate Defender.

Some variants of Trojan-Downloader.Zlob have a backdoor functionality giving a remote attacker the ability to control and use the infected machine for malicious purposes.

Following closely behind is Trojan.gen and LdPinch Trojan, also a critical threat.

Webroot identified 007 Spy as the month's most common system monitor. The malware records the activities of a computer, but is considered a critical threat only if the user is unaware of it being installed on their computer.

"Spyware has become increasingly complex and sophisticated," said Peter Watkins, chief executive at Webroot.

"The technology behind spyware is so far advanced, and is moving so quickly, that the manual detection methods used by many security companies and freeware providers cannot keep up."