QUESTION When clicking on a hyperlink in Mozilla Firefox I often get a warning that the ‘URL is not valid and cannot be loaded’, even when I know it’s genuine. My PC runs Symantec Norton Internet Security 2010 and a-squared Anti-Malware, but I can’t find any settings to adjust to prevent this happening. Martin Perry

HELPROOM ANSWER You shouldn’t be running both Norton and A-Squared, Martin. Running two security products that provide the same function simultaneously can cause them to clash, producing random behaviour problems.

If you’re unable to uninstall A-Squared using Add/Remove Programs, you may need to boot the PC into Safe mode first.

The error message you’re receiving is likely to be caused by a Firefox browser add-on (although the Google Toolbar is often wrongly blamed). In Firefox go to Tools, Add-ons and disable all entries. If all is well after restarting the browser, enable each add-on in turn to identify the culprit.

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