You need to make sure pythonwin knows where the ArcGIS python packages reside. A simple way to do this would be to create a text file called arcgis.pth (or anything you want with ".pth" as the extension) in the Lib\site-packages directory of your pythonwin installation. The contents of the file should be these 3 lines (modified as needed to reflect the paths of your ArcGIS installation):
C:\Program Files (x86)\ArcGIS\Desktop10.0\bin
C:\Program Files (x86)\ArcGIS\Desktop10.0\arcpy
C:\Program Files (x86)\ArcGIS\Desktop10.0\ArcToolbox\Scripts
Once that file is in place, pythonwin should be able to access arcpy
successfully.
(Note that I wrote this based on my own ArcGIS 10.2 installation. I believe the directory structure was the same for 10.0, but if anybody can confirm this, that would be helpful.)