SearchScan uses security firm McAfee's SiteAdvisor technology to warn users about "potentially risky sites".

The service, which is switched on by default, produces an on-screen alert.

"Our goal is to protect users by allowing them to make a more informed decision about the sites they visit," said Yahoo's Priyank Garg.

Rival firm Google introduced similar technology in 2006.

Yahoo's service will warn users about three types of risk:

• Browser exploits: Sites that can harm a user's computer or install malware simply by visiting the site. Any such sites or pages included in McAfee's data will be removed from search results automatically.
• Dangerous downloads: SearchScan will display warnings next to search results for sites that offer potentially dangerous software, such as viruses, spyware or adware.
• Unsolicited e-mail: SearchScan will alert users to scanned sites that send unsolicited e-mails or inappropriately share e-mail addresses with third parties.

Viruses, spyware and adware programs are often "hidden" inside innocuous-looking programs such as screensavers and toolbars.

Industry analysts IDC estimate that 67% of all computers have some form of spyware installed without a user's knowledge. – BBC News