WPF has the SystemParameters class that exposes a great number of system metrics. On my computer I have noticed that a normal window has a title that is 30 pixels high and a border that is 8 pixels wide. This is on Windows 7 with the Aero theme enabled:

However, SystemParameters return the following values:
SystemParameters.BorderWidth = 5
SystemParameters.CaptionHeight = 21
Here I have disabled the Aero theme:

Now, SystemParameters return the following values:
SystemParameters.BorderWidth = 1
SystemParameters.CaptionHeight = 18
How do I compute the actual observed values by using SystemParameters?
For a resizable window you need to use a different set of parameters to compute the size:
These sizes will change when you modify the theme.