ISPs say the spam sent by zombie PCs is their biggest challenge and is the biggest threat to the security of their business users, according to the results of a survey published this week by StreamShield Networks, a provider of managed internet security.

ISPs said they were worried by the potential "disruption to service that spam zombie machines can cause". This issue was raised by 96 percent of respondents. With zombies thought to be present on a large number of home PCs and propagating up to 70 percent of all spam, this concern is understandable.

Three other challenges tied for second place and were identified by 92 percent of ISPs. These were: the difficulty of clearly differentiating their services from those of their competitors; the problem of finding new sources of revenue; and the challenge of reassuring subscribers of the security of their services.

Geoff Bennett, director of Product Marketing at StreamShield, said, "ISPs can work to meet these challenges quickly, simply by deploying new technologies, which already exist in the marketplace. The issues of differentiation, finding new revenue sources and reassuring customers of security can also be addressed through third-party offerings.

Bennett added, "Home users and business customers are taking a more proactive approach to reviewing service levels from suppliers, and considering monthly costs of subscription. ISPs are unfortunately under more pressure than ever before to improve services if they want to maintain their current revenues, let alone increase them."

The survey was conducted during a London Internet Exchange (Linx) event in London. Fifty UK ISPs responded.