That depends on what you mean by accurate, and even then they pretty much suck. Clocks are "inaccurate" in a few ways. One of these ways is very short-term: variations in exactly when the clock edges arrive at frequencies near the clock frequency. Others of these are long-term: how far the clock is from it's target rate over decades. Between these there's a whole spectrum of different noise frequencies.

The oscillators on your motherboard, with very basic temperature correction, very basic calibration and only very basic quality control don't tend to do very well on any of these measures. They tend to do better on the high-frequency measures of clock badness, and worse on low-frequency measures, but they aren't stellar in any area. The end-to-end time system, including NTP and software-level drift correction, does an OK job in some circumstances. It does a horrible job in others.

If you go through life assuming your computer's local clock isn't literally out to get you, you're going to be disappointed.