Web surfers are accustomed to seeing a 404 error message when they try to reach a website that is not available. But now hackers are using that common occurrence to their advantage by creating fake sites containing the error message to load spyware and adware, security researchers said today.

One particular site - http://404dnserror(dot)com - "tries to install an ActiveX control and the installation message communicates that page is not available as it's blocked by adware/spyware," said Bhaskar Krishna on the McAfee Avert Labs blog today.

The site asks users to install security software called "System Doctor" to remove the adware and spyware that is supposedly preventing the site from opening. But System Doctor is part of WinFixer, adware that uses deception to encourage people to purchase often rogue software.

According to Symantec, WinFixer is software that "may give exaggerated reports of threats on the computer. The program then prompts the user to purchase a registered version of the software in order to remove the reported threats."

Krishna said users should be wary of bogus websites reporting error codes as they surf the internet.