I'm running Windows XP Pro with SP2.  Somewhere along the line
(probably due to a software installation) Windows Explorer decided to
stop displaying the little dotted lines that connect nodes on the tree
list, making it difficult to scan the folder structure.  Does anyone
know how to fix this?

save your money mate - I would be charging you but Google are not
taking on any more researchers so i will tell you for free.

Open any explorer window > Click Tools > Folder Options > Click View
Tab > Untick 'Display simple folder view in Explorer's Folder List'
option > Apply

Brilliant - thanks!  I never would have found that in a million years. :)

(Note to self for future reference: Microsoft appears to have
redefined the word "simple".)