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For the record, I'm fairly experienced as a computer user, and mostly competent. As such, I would never uninstall one working security product to install another unknown untested beta product on a production system, even if it is my bleeding edge desktop. I definitely expect some performance problems running multiple security suites. I've run as many as 3 simultaneously before without such hanging issues.
Which exact McAfee product do you have installed on your pc? In order to prevent issues and clashes, AV programs usually detect an existing security solution and don't allow to continue with installation until the
current AV program is uninstalled completely. This check is also performed by ESET products, including v7.