I thought Acad did it by default. If you look at the status bar after starting e.g. the line conmmand you will see relative co-ordinates displayed. When no command is active, absolute co-ords are displayed.
System Variables
COORDS
Type: Integer
Saved in: Registry
Initial value: 1
Controls when coordinates are updated on the status line.
0 Coordinate display is updated as you specify points with the pointing device
1 Display of absolute coordinates is updated continuously
2 Display of absolute coordinates is updated continuously, and distance and angle from last point are displayed when a distance or angle is requested
AutoCAD displays the current cursor location as a coordinate on the status bar at the bottom of the AutoCAD window.
There are three types of coordinate display available:
Dynamic display: Updates the coordinate values as you move the cursor.
Static display: Updates the coordinate values only when you specify a point.
Distance and angle: Updates the coordinate values as you move the cursor and displays the values in the format distance<angle. This option is available only when you draw lines or other objects that prompt you for more than one point.
When you edit objects, you can cycle through the three types of coordinate display by pressing F6 or CTRL+D. You can also right-click the coordinate display on the status bar to access the display options from a shortcut menu, or you can set the COORDS system variable to 0 for static display, 1 for dynamic absolute display, or 2 for distance and angle display.
You do have co-ords turned on don't you ? ~ you can turn them on and off by double clicking on the bottom left area of the status bar