Mac OS X - rmdir fails with "Operation not permitted" for a folder created by a PC on a removable drive

Mac OS X - rmdir fails with "Operation not permitted" for a folder created by a PC on a removable drive

I have a problem (using Mac OS X 10.5.8) with the access rights of a folder that was presumably created by a virus on a disk-on-key drive when I used it with a PC. I can't remove the folder or change it's name. In Finder's Info window the Lock box is unchecked and uncheckable - if I try to check it it flips back to off.