I'm just wondering if I can trash these folders even if I'm adding hotfixes to my install. This may have already been covered in some other thread a while back, but my initial search didn't find anything.

I shall continue my search unless someone wants to shorten the task for me

Yes.

all you need is the i386 dir ... you may still remove some other dirs from it (win9x dirs are very safe to remove too)

unless you need deployment tools (sysprep) from SUPPORT, they are of no use to you.

Which of those folders can you remove and still keep the ability to upgrade or repair? Both would be done from the same version of XP, but Windows still calls it an upgrade, anyway..

winntupg is required for the upgrade especially if you'd install recovery console ... win9xmig & win9xupg are for upgrading from lower OSs

Excellent. The Win9x* folders alone are 37MB.

Gone!