Just because you are an admin doesn't automatically mean you have rights. Are you running PowerShell as an elevated user (UAC)? Have you checked the permissions of the files in question just to be sure? Are the files in use perhaps? Although if that was the case I would expect access is denied.
Have you tried also using -recurse and -force to be sure it gets them all? For what its worth rmdir is an alias for Remove-Item
Remove-Item "c:\xampp\htdocs\prestashop\dfs" -Recurse -Force
You will see, from TechNet, that -Force
Allows the cmdlet to remove items that cannot otherwise be changed, such as hidden or read-only files
