Internet Optimizer, also known as DyFuCA is a spyware program which redirects Internet Explorer error pages to advertising pages. When users follow a broken link or enter an erroneous URL, they see a page of advertisements. However, because password-protected Web sites (HTTP Basic authentication) use the same mechanism as HTTP errors, Internet Optimizer makes it impossible for the user to access password-protected sites.
It is classified as a "Browser Helper Object" or BHO, and as such, it loads whenever MS Internet Explorer starts and is not detected by firewalls or anti-virus software, as it is seen as a legitimate part of the application. It is also classified as a "downloader" which can download, install and run software on the victim's computer without their permission.