Ryan: forget about getting anything really exact when running under traditional operating systems (that includes linux, too). Forget about turning off interrupts. Forget about any “guarantees”.
Basically, the best you can do is boosting your thread priority to your OS equivalent of “realtime” (requires admin/root), giving away the reminder of you current timeslice, and then running your code… usually, for routines that take less than a second, you’re going to run the routine N times, and either do an average, or record min/max/avg times… or even recording all times, and reporting mean time as well.
If you want anything more accurate than that, get a profiler (AMD CodeAnalyst or intel VTune).
Oh, and remember to “burn some rubber” before doing the test, so you’re sure the CPU is not in reduced-speed power saving mode.