[Xurinos]

http://en.wikipedia.org/wiki/Time_Stamp_Counter

"The issue has two components: rate of tick and whether all cores (processors) have identical values in their time-keeping registers. There is no promise that the timestamp counters of multiple CPUs on a single motherboard will be synchronized. In such cases, programmers can only get reliable results by locking their code to a single CPU."

The entry also mentions that hibernation can affect the counters. I wonder if power savings implementations that speed up or slow the CPU could also have an effect.

[tptacek]

Yes. The TSC, which counts cycles, is what WinAPI reads with GetTickCount(), and is not the same thing as the HPET timers.