I have parcel and buildings layers.
In their attribute tables I want to replace "NULL" values with numbers from 1 to 2000, in "Parcel ID", or "Building ID" field.
How can I do this in ArcGIS Desktop?
Here is a code block for the Field Calculator that will do what you require.
Accumulative and sequential calculations
This code sequence is taken from the ArcGIS 10.0 Desktop Help: Calculate Field Examples
I would highly recommend that you browse through the help documentation in that file and in the other help sections for ArcGIS, as there are many useful examples and code samples to pull from.
Enter the code above, into the Field Calculator as shown in the screen shot below: You would enter the code into the sections as detailed below: