How much clock drift is considered normal for a non-networked windows 7 PC?

I have a stand-alone PC with no network connection running Windows 7 (64bit) and the system clock loses about 1 minute per week. Is this level of drift considered "normal" or is it a dodgy RTC or a Windows configuration issue or something else?

What is the expected accuracy (roughly) of a PC clock?

EDIT: The PC is never switched off.

I would also like to know what is the typical ("expected") clock drift of standard RTCs found in common PC hardware. The NTP FAQ from www.ntp.org provides some information on the topic:

• 3.3.1.1. How bad is a Frequency Error of 500 PPM? says that a 500 PPM error corresponds to a drift of 43 seconds per day, and "Only poor old mechanical wristwatches are worse."

• From 3.3.1.2. What is the Frequency Error of a good Clock?:

  I'm not sure, but but I think a chronometer is allowed to drift mostly by six seconds a day when the temperature doesn't change by more than 15° Celsius from room temperature. That corresponds to a frequency error of 69 PPM.

  I read about a temperature compensated quartz that should guarantee a clock error of less than 15 seconds per year, but I think they were actually talking about the frequency variation instead of absolute frequency error. In any case that would be 0.47 PPM. As I actually own a wrist watch that should include that quartz, I can state that the absolute frequency error is about 2.78 PPM, or 6 seconds in 25 days.

  For the Meinberg GPS 167 the frequency error of the free running oven-controlled quartz is specified as 0.5 PPM after one year, or 43 milliseconds per day (roughly 16 seconds per year)

• 8.3.1.1.1. How accurate is the CMOS clock? mentions (typical?) frequency errors in the range of 12 to 17 PPM for one specific machine.

Most of my machines are around 20 ppm off, which is about 12 seconds per week. So you're seeing about 5 times the average. That's unusually high, but not so high that it means something is necessarily wrong.