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There are two ways to fix this problem:
1. Check the "Monitor clicks with hotkey only" box in the Download Express Options dialog, Integration tab and click the OK button. Download Express will catch clicks with depressed hotkey combination only. So, your antivirus updates will go well.
2. Add the exact exe-files names to the Download Express Registry exceptions list using the Registry Editor Windows application. Please write me if you want to use this way, I will describe it further.