Does a personal computer have just one clock and where is it?
There's several clocks:


real-time clock (RTC) that is battery backed
system reference clock from which other clocks are defined
external (internet) clock


The RTC (sometimes called the 'CMOS', since it used to be part of the battery-backed RAM) is a small, low-powered timekeeping clock that runs on a low-frequency watch crystal. When the system is off, this clock is kept alive with a small battery such as a coin cell, or in some cases, a supercapacitor. This defines the wall-clock time when the system starts up, and also when there isn't a reliable external time reference available.

The system reference clock is a crystal or oscillator that feeds the motherboard chipset and CPU. Typically the crystal is tens of MHz; it can be multiplied up for distribution (details are chipset-dependent.) This clock in turn drives the various PLLs that define the processor, memory and I/O clocks. The CPU bases its time-tick on this clock when it's running, and will at certain times synchronize its computed wall-clock time to a known-good value from an external source.

Finally, computers connected to a network will get wall-clock time from a traceable source, often using a protocol called NTP, or Network Time Protocol. More about that here: http://www.ntp.org/

Neither the RTC or system reference clocks are especially accurate, having typical crystal frequency errors in the 10 ~ 50ppm range. NTP provides an authoritative, atomic-clock reference standard which is used not only to correct the drift on system time-tick clock, but also to update the RTC.