You have to be signed in as an Administrator to install software. If you aren't then you should save the installer to your desktop and then right-click it and select 'Run as Administrator'.
Another possibility is your version of Internet explorer. make sure it is the latest (IE9 for Vista/Windows 7 and IE8 for XP). McAfee uses IE regardless of your default browser settings.
If you already have it open it and go to Tools/Internet Options/Advanced and click Reset to put it back to default settings. Click Apply and OK.
Close and reopen IE and then go to Tools/Manage Add-ons and re-enable any add-ons you need.
Then try the installation again.
Having the exact same ploblems while using Windows 7. I've tried everything..
The best alternative to me posting workarounds that probably wont work is to contact Technical Support online chat, they have ways and means of getting things to work quickly.
Linked under Useful Links at the top of the page.
An un-updated browser could be the cause for the java script error while installing McAfee programs.
Please follow the below steps and retry the installation ;
Open Internet explorer – click on Tools – Click on Windows Update and install any available update from the site
Run the McAfee removal tool from here http://download.mcafee.com/products/licensed/cust_support_patches/MCPR.exe and restart the PC
Login to your McAfee account page from here and retry the download .