Well I don't know the in's and out's of how XP works decides what display options should be available for any particular folder, but here's the fix.
Right click on the naughty folder, select "Properties", select the "Customize" tab.
Now select "Photo Album (best for few files)" from the drop down selection for the "Use this folder type as template:" field, click OK and the folder will display in Filmstrip mode.
You can also select "Pictures (best for many files)" if you wish, but it mean when you click OK the folder will display as Thumbnails. The Filmstrip option will however also be available. The option selected by default is, apparently, the only difference between those two templates. Makes one wonder why they bothered with two templates (with confusing braketed text) when one would have sufficed!