Nearly three in four companies are infected with some form of malware – including those with up-to-date security software installed, according to a survey by anti-virus specialist Panda.

Nearly one in four home computers are similarly infected, according to the analysis of more than 1.5m machines.

The number of malware variants is increasing exponentially as criminal malware authors seek to bypass security software, the company said.

Most infections are through websites and most are trojans that can be used to pull in other code, such as spyware or spam launchers. Often the code is targeted only at a few machines and new variants are produced each day.

The result is that traditional methods of containing the problem can't keep up, said chief technology officer Iñaki Urzay. Variants appear quickly and to keep signatures of all of them would eventually require tens or hundreds of megabytes of data on each user's machine.

Panda has developed a new system called Collective Intelligence that draws real-time information on new variants using existing industry notification channels and its customers' machines as virus sensors.

A tiny client on each machine notifies Panda of suspicious code and it is analysed on the company's servers rather than the infected PC. Panda then blocks the operation of the malware, which is usually fairly easy with Trojans.

To prove its point about infection levels Panda has challenged users and companies to check their machines at a site called Infected or Not?. Home users who find they are infected are automatically entered into a lottery with a prize of one iPod Nano a week.

Any infection-free company gets a chance to win £5,000 for a charity of its choice. The closing date for both is mid-July.

Pandalabs technical director Luis Corrons said Russian criminals are the leading malware distributors and openly offer their services for sale on the web, where credit card details can be bought for as little as $2.