If the directory is named something like "9470bb12eb8a4f344765a27236478e41c5" or something like that I believe those are reminiscent of Service Pack installations are are supposed to be automatically deleted. If they weren't automatically deleted (as probably in your case) they can be safely deleted. Have you installed a Service Pack recently?
To delete these folders, first take ownership of the folder and then you can just delete it.
What do you mean by "how is it possible to restrict a folder this way"? You mean to... duplicate the "access denied" effect? To do that I think you can just take ownership of a folder and remove permissions for the user you want to deny access to...
UPDATE: To take ownership in XP Home, start in safe mode and log in as admin to get access to the security tab... and then right click folder->properties->security tab->advanced->owner tab, then select your user, "replace owner on subcontainers and objects", etc and then you can delete. You might have tried this already... in that case I wouldn't know much else to help you :)