Update on this issue:
I used "RegShot" from SourceForge (
http://sourceforge.net/projects/regshot/ ) to determine which key(s) change in the registry when I change the Active Border Width in Windows XP. This Active Border Width setting also changes the width of the divider between the Folder pane and File pane in Windows Explorer on XP.
The setting that changes is:
HKEY_USERS\S-1-5-21-123456789-123456789-1234567890-1003\Control Panel\Desktop\WindowMetrics\BorderWidth
(I changed the numbers after S-1-5-21-, in case that would identify me to Microsoft).
The Active Border Width can be changed in XP by going to Control Panel / Display / Appearance / Advanced. Use the drop down box to select "Active Window Border".
When the border is width "1', the key value is "-12".
When the border is width "2', the key value is "-24".
When the border is width "3', the key value is "-36", etc.
This key is also present in the registry of Windows 7, so I was hopeful that I could change it to produce the same effect in Windows Explorer on Windows 7. No such luck.
I also discovered that there is a way to adjust this setting without going into the registry: Control Panel / Display / Personalization / Window Color. Then use the drop down box to select "Active Window Border".
When the border is width "1', the key value is "-15".
When the border is width "2', the key value is "-30".
When the border is width "3', the key value is "-45", etc.
So, Windows Explorer in Windows 7 does not use the Window Border setting when drawing the line between the Folder pane and the File pane.
But, guess what DOES use the Window Border setting... "regedit"! Yes, they screwed up Explorer but, thankfully, kept their sticky little fingers away from the "regedit" code. So "regedit" has a nice thick / adjustable divider which is easy to grab, but Explorer - which needs it much more - doesn't.