When I run Clockres on my Windows XP workstation, I receive:

ClockRes - View the system clock resolution
By Mark Russinovich
SysInternals - www.sysinternals.com

The system clock interval is 15.625000 ms
The ClockRes page contains the following introduction:

Ever wondered what the resolution of the system clock was, or perhaps the maximum timer resolution that your application could obtain? The answer lies in a simple function named GetSystemTimeAdjustment, and the ClockRes applet performs the function and shows you the result.