What do you mean by "illegal characters"? Because if they were illegal they woldn't be in the filename in the first place (Windows woldn't have allowed it). As for the folder being in use by another process, make sure you don't have anything else opened that may be reading or writing the files contained in that folder. If you're not sure, try closing every other program before renaming the files with Bulk Rename Utility. As a last resource, reboot your computer and then go directly to Bulk Rename Utility. There shouldn't be anything locked now.