If your browser starts popping up ads on sites that never use popup ads themselves, such as Google, fails to reach some sites altogether or sends you to completely different content, there is a good chance your computer is infected by spyware and/or adware. These programs are harmful, invasive of your privacy, and generally awful.
Frequently, users make innocent but uninformed decisions to accept dialog boxes that ask whether you want to trust software from a company you have never heard of, or a similar message. Spyware can also sometimes be installed automatically, especially if you have not used Windows Update to install all Microsoft fixes for your operating system. Update, 2004-09-05: Microsoft Windows XP Service Pack 2 does offer some protection against new spyware installing itself on your computer, however if your computer is already infected, you must use SpyBot or AdAware to clean up the mess first; see below.
If you suspect you are suffering from one of these programs, you should use SpyBot and/or AdAware, two excellent pieces of freely available software, to clean up the mess. These utilities do a fine job of removing spyware and other unwanted garbage that has been installed on your system. However, neither of these programs automatically updates itself to recognize new types of spyware, so be sure to click on the "fetch updates" button in either of these programs first before starting a search for spyware on your computer.
You should also consider running an alternative browser, especially if Microsoft has discontinued support for your version of Windows, making new security fixes potentially unavailable. I suggest the free and fully functional Firefox browser, which has many other advantages. Windows users should also consider the exceptionally fast web browser Opera. Formerly shareware and/or adware, Opera has been completely free of charge since late 2005. MacOS users can also use Apple's Safari browser; this is strongly recommended as Microsoft has announced an end to future development of Internet Explorer on MacOS. All of these alternatives are less prone to known security problems than Microsoft Internet Explorer, although you must keep any browser up to date with the latest version available to avoid security problems in the future.
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