The answer to your question is surprisingly complex, and I probably will not do it justice, but can at least point you in the right direction.
Most desktop multitasking operating systems have a "slice time". This is the smallest amount of time the operating system will give to a task or thread before it switches to another task or thread. So if you have a single processor core and no hyperthreading (one physical thread), and are running three active operating system threads, the operating system will switch between the three threads with an interval equal to the slice time. The default slice time for Windows XP is about 15ms, which corresponds pretty well to the resolution you are seeing. You can change the default slice time, although I cannot remember how (a net search will probably find this for you pretty quickly). This may effect your processor efficiency, however, since the processor now has to do a lot more thread switching.
In addition, you will see different behavior from the Tick Count (ms) primitive and the Get Date/Time in Second primitive. The former usually has better performance.
Buried in the settings for Windows XP, there is an "optimize for background services" or something similar. LabVIEW works best when this is enabled. The default for XP is "optimize for foreground". This may fix your problem.
If you want higher resolution, you can call into the performance timers of your processor using the Windows KERNEL32.DLL. There are some VIs to do this posted here, along with further discussion on timing. Note that higher resolution does not get around the slice time issue.
If you need further information, let us know.
