No, what you see here isn't the source code to Windows XP Beta 2, nor do I even have access to source code. This is the source tree layout of the Windows XP Beta 2 sources, complete with file names. It also contains the Kernel, which implements NTs synchronization mechanisms. An assert statement is simply a run-time expression that should be true. If it isn't, then some program variable has attained a value considered impossible by the program and the assert statement flags the condition in some way visible to a programmer.
Developers sprinkle code with assertions to catch faulty assumptions and programming errors.
RtlAssert prints the expression, file, and line number information as a debug output, and causes a debug break so that a developer can examine the conditions that lead to the assertion failure from inside a kernel debugger. The file name parameter causes the source file's file name to be embedded in its image file, and it can be extracted with a string-search utility like my own Strings .