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Internet Explorer has JavaScript enabled by default, and comes with all Windows-based computers from Windows 98 onwards.
Mozilla and Mozilla Firebird have JavaScript enabled by default... I have no idea what Opera's defaults are, but I would take a guess that it would be enabled.
It's pretty safe to assume that the great majority of users will have JavaScript enabled, but you should never control navigation or critical website functions by JavaScript, for those who purposefully disable it, or do not use a JavaScript-capable browser (such as, I believe, Lynx) or perhaps a text-based browser, which are popular with the blind Internet users.