Try running a scan using the free version of THIS software, update before running and let it remove anything it finds. Reboot immediately if it asks you to.
You may have to uninstall that software to allow McAfee to install. But reinstall it later for future use.
Next download the MCPR McAfee software cleanup tool from Useful Links at the top of this page and reboot. Try the installation again.
If it still fails I need to know what operating system and service pack this is, what installed RAM (memory) you have and how much free hard drive space is left.