The global spyware plague has reached epidemic proportions, with the cost to global PC users set to rocket by 2,400 per cent over the next four years.

According to newly published research from IDC, the need to identify and eradicate these parasitic programs will drive anti-spyware software revenues from $12m in 2003 to $305m in 2008.

The analyst firm reported that spyware infects millions of computers with the purpose of stealing personal information, enabling identity theft, tracking online activity, and selling information back to anyone willing to pay.

Although not always malicious in nature, IDC noted that spyware still causes significant damage to legitimate software, network performance and employee productivity.

An indirect cost of spyware identified by the IDC report is that it crosses the boundary between security and system management by deluging help desks with complaints about pop-ups, application failures and poor PC performance.

At worst, spyware's ability to track keystrokes, scan hard drives and change system and registry settings is a tremendous personal and enterprise security threat leading to identity theft, data corruption and even theft of company trade secrets, IDC warned.

Brian Burke, research manager for security products at IDC, said: "Today, more malicious spyware can easily infiltrate corporate firewalls. These programs make their way into the corporate intranet under the guise of less threatening network traffic, and can wreak havoc."

The report found that spyware is often bundled with legitimate programs, allowing it to pass easily through firewalls. IDC estimated that 67 per cent of all computers (mostly consumer) contain some form of spyware.