What is your operating system and service pack and are you totally up to date, including parts of the system you may not use? For instance McAfee uses Internet Explorer for display purposes so make sure that you have IE9 installed if Vista or Windows 7, IE8 if XP and make sure that Java is installed plus ensure that the browser & all add-ons are totally up to date.
Did you have any security software installed, if so remove it and use any removal tool thay may have.
If you had McAfee installed, uninstall it via Control Panel, then run the MCPR cleanup tool (listed under Useful Links at the top of this page), then reboot and run the PreInstall Tool, then install the software from your online account.
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