From GRC|SecurAble: Determine Processor Security Features|
How do 64-bit instructions
help with security?
64-bit-capable processors have the ability to run the 64-bit versions of Microsoft's substantially more secure XP, Windows 2003, and Vista operating systems. Those operating systems are more secure because Microsoft, having learned many lessons from mistakes in the past, made the firm decision to lock-down their 64-bit OS kernels. The 64-bit Windows kernels actively police themselves to guard against many rootkit-style and other kernel attacks that have caused so many problems for users of the 32-bit Windows operating systems.
These advanced kernel-protection technologies cannot be ported back into current or even future versions of Microsoft's 32-bit operating systems because doing so would break so many existing programs and drivers as to make the system impossible to use. Microsoft knows that one day the personal computing industry will have moved over to 64-bit operating systems much as we all once moved from the 16-bit based systems to 32-bits.
SecurAble indicates by displaying either a 32 or a 64 whether the system's processor has the 64-bit instructions or extensions necessary to run 64-bit versions of Microsoft's present and future operating systems.