TortoiseSVN seemed to be slowing down my Windows Explorer a whole lot. So I scouted around and was reminded (via google) that there is a feature available via the TortoiseSVN Settings dialog where you can exclude certain paths from the Icon Overlay treatment.
So I excluded the whole of the D: drive (by putting the line D:* in the "Exclude paths:" area), and then included back my working folder (by putting D:\Petert\PWC* into "Include paths").
But now what happens is that the folders in my working folder get the Tortoise overlays, but not the files!