A modern PC actually has several clock crystals; there's usually at least a high-frequency one that generates the main CPU/chipset clock, and a much lower frequency one (32.768KHz) for the realtime/battery-backed clock. On the motherboard I'm currently using I see two more, one for the Ethernet chip and another on the audio codec. They would all drift somewhat with temperature, but I think for this system at least, using the RTC clock for coarse timing would probably have the least temperature variance as it's far away in a corner of the board and at the bottom, away from any heat-producing components.
I suppose you could also use some sort of GPS-disciplined oscillator too.
I suppose you could also use some sort of GPS-disciplined oscillator too.