Build hardware. Map some critical sections of your software onto a peripheral device accessed via USB. USB device not present? Software no worky.
It's security by obscurity where obscurity includes "building hardware".
Depending upon your level of paranoia, you can include flash-programmed FPGAs with a "security bit" (example) into your USB peripheral design, to make reverse engineering a couple orders-of-magnitude more difficult.
Customer is a hacker. He can patch the USB code into the program, package it nicely and then re-sell it courtesy MGJ Archaic Technologies, Inc.
The problem with software protection is that in order for software to run, it must run in native assembly code without any encryption. Thus, by definition, any protection scheme can be bypassed by looking at the instructions that are loaded in memory at run time.
Man, I too old for this crap...It brings up memories...
