If unwanted software gets into your system and can run with your administrator credentials, it can also access anything it wants without restriction - both file system and registry. Restricting security so that day-to-day tasks can be done but any significant changes require administrator intervention or permission reduces the likelihood that unwanted software can make damaging changes. For example, leaving UAC on means Internet Explorer runs in protected mode, which sandboxes the IE session and prevents malware infecting your system. Microsoft have found that Vista is 60% less likely to be affected by IE drive-by infections than XP was.
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