Not exactly answering the microsecond issue, but an interesting utility for keeping your computer's time correct can be found at: http://www.boulder.nist.gov/timefreq/service/its.htm
They go on to explain:
On most Win 3.x, 9x, 2000 and Me systems, the time cannot be set more accurately than to the nearest second. It is therefore possible that the time of your system can be wrong by up to +/-0.5 seconds even immediately after you adjust your clock based on data received from one of our servers. In addition, the clocks on many computers can gain or lose several seconds per day, so that even a clock that is set correctly will not stay that way for very long. Windows NT has more sophisticated software support for the clock, but the hardware is usually not any better. If your applications requires that the time on your system must be correct to within +/-1s at all times then you will probably only be able to achieve that level of performance using Win NT and a continuous connection to the Internet. It will be more difficult to realize using other versions of Windows, and you may find that even Win NT does not completely satisfy this requirement.