Your processor is a 64-bit chip that runs in 32-bit compatability mode.
For it to totally utilise the 64-bit stuff, everything in your computer needs to support 64-bit too -- the chipset (in addition to processor), the operating system, drivers, everything.
Most likely your operating system, Vista, is not 64-bit. Most aren't. As such, you can't use it as a 64-bit system.
McAfee will still run fine, as long as it supports Vista.
64-bit is sometimes sold as a way of getting your computer to run faster. It's not really what it's about. 64-bit means that you can use much more RAM than you currently can. So, unless you require a use of your computer that demands more than 4GB RAM, you don't need 64-bit.
As such, don't worry about it. Ignore the 64-bit stuff. If it's vista compatible, it'll work.