If I recall correctly, Blitz doesn't call QueryPerformanceCounter & QueryPerformanceFrequency to determine the elapsed time, so this shouldn't be an issue.
Microsoft says to use QueryPerformanceCounter & QueryPerformanceFrequency instead of RDTSC.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/directx9_c/Game_Timing_and_Multicore_Processors.asp