Make sure you run outside of the IDE. That is key.
-edit- I LOVE SLaks comment. \"The amount of misinformation in these answers is staggering.\" :D
I think the problem is your timing code in C++.
From the MS docs for __rdtsc:
__rdtsc
Generates the rdtsc instruction, which returns the processor time stamp. The processor time stamp records the number of clock cycles since the last reset.
Perhaps try GetTickCount().
GetTickCount()