Guy, who knows why changing one byte in a program disables it?
I have an EXE. I open it with CreateFile and ReadFile in to buffer. I change TEST EAX,EAX to XOR EAX,EAX (85C0-->33C0), then I write it back into this file. It has the same size. But it runs to this place and dies with "Insufficient memory".

These instructions give far not equal results.
Probably further code depends on this result and works improperly.