OSLO, Norway — Many American online computer users are unaware that choice of browser affects Internet security, and few switch browsers even when they know the risk, a Norwegian study said Monday.

The Oslo-based browser-maker Opera Software ASA, which touts its own browser as being one of the most secure, released a survey of 2,835 online users in the United States, which indicated that only 51 per cent of what it called the "adult online population" were aware that the type of browser can affect a computer's vulnerability to malicious software, such as viruses and spyware.

The poll, first released to The Associated Press, also showed that only 11 per cent of those asked said they had switched browsers for security reasons. The survey was conducted in March 25-29 by the Harris Interactive polling group and had a margin of error of about 5 per centage points.

Malicious software often targets a specific type of browser, and generally those that are the most widely used. The poll said that 49 per cent of those asked did not know that their choice of browser can make a difference, including 17 per cent who thought it had no effect.

Although a small percentage said they had switched browsers for security reasons, 66 per cent said they would consider a change if it would improve security.

"Changing to a more secure browser is one of the simplest ways for Web users to make surfing safer and minimize the risk of falling victim to virus, spyware or 'phishing' attacks," said Opera's chief technical officer Haakon Wium Lie, referring to various techniques of tricking Internet users to hand over personal information, such as credit card numbers and passwords.

The tiny Norwegian browser company claimed that Internet security upgrades were a key part of its Opera 8 version, released last month. Among the features is a special window that rates the security of the page visited on a scale of one to three.

Opera commands less than 0.2 per cent of the Windows market, far behind the industry leading Internet Explorer from Microsoft Corp. and various open-source browsers, such as Mozilla Firefox.