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It's one of the hazards of browsing the web: you open a new page and all of a sudden an extra window pops up and obscures what you're trying to read. These windows are, not surprisingly, called 'pop-ups' and they may be annoying or useful, depending on the context.

The first use of pop-ups - and still probably the most common - was to display ads. One variant on these is known as 'pop-under' and loads a new, hidden window for you to discover later. A second use, found on some financial sites, is to open a secure session. A third, usually found on publication sites, asks users to log in, register, or complete a survey. Often these are a pop-up variant known as a 'hover window', which continues to block most of your screen even if you scroll downwards.

Pop-up windows often lack the controls familiar from your main browser windows. Advertising and secure session pop-ups may be void of menus, toolbars, status bars, or other navigational aids, and resist being moved or resized. You should always be able to close them like any other. For secure sessions the clean-screen approach helps control your path through the session to avoid disruption. (For ads, it's just annoying.)

Hover windows may have an 'x' in one corner or the word 'close', or something similar such as 'no thanks', at the bottom that you can click on to close them. Those blocking access to content, however, are designed to remain in place unless you enter a valid user name ID and password.

A final type of pop-up window comes from spyware installed on your computer that is designed to trigger pop up ads and bogus warnings. To get rid of these, you will need a reputable anti-spyware program, perhaps as part of anti-virus software.

Many users find pop-ups annoying. As a result, every time someone develops a new way of creating pop-ups, someone else responds by developing a blocking program for it. There are pop-up blockers for every type of web browser, either built-in or available as an add-on.

Exactly how you disable pop-ups depends on which browser you're using and what type of pop-up it is. In Firefox, for example, the pop-up blocker can be found by clicking on the 'Content' tab that you'll find if you choose 'Options' from the 'Tool' menu. In Internet Explorer, click the 'Tools' button, then select 'Internet Options' and click on the 'Privacy' tab to find the pop-up blocker.

As technologies evolve these built-in tools may no longer be enough to block all pop-ups. Therefore, it helps if you know that creating pop-ups requires a bit of program code that runs inside your browser to open the window and load the content that appears in it.

Most often, this program code is written in a language called JavaScript which you can block in your browser's settings via the Tools menu, but it's quite fiddly.Ìý

In Internet Explorer you will need to go to the Tools menu, select Internet Options... then click the Security tab. You'll then to clisk Custom Level... then click on Disable in the Scripting section. You will then need to restart your browser. In Firefox, select Options... on the Tools menu, then select the Content tab. Uncheck Enable Javascript before clicking OK to finalise the selection.

For more complex pop-ups, like hover windows, try searching for a blocking tool for your browser, but don't expect it to be perfect.

Wendy M. Grossman is a freelance technology writer and author living in London and is founder of The Skeptic magazine.