That's interesting, thanks for the information.
No encryption was (purposely) placed on these files. On the Mac OS X end, I dragged a folder of files into an email message I was addressing to myself; the system notified me it would be packaging them as a zip file.
When I received the email on my Windows 7 PC, I saved the zip file to the desktop, then used Windows Explorer's "Extract All." Twice during the extraction process, this error message appeared: "An unexpected error is keeping you from copying the file. If you continue to receive this error, you can use the error code to search for help with this problem. Error 0x80070057: The parameter is incorrect." However, the extraction process continued, and all of the files are accessible normally.
The error messages appeared when extraction was being done on a pdf file that had a filename including accented letters -- I wonder if this threw a wrench into things in some way?
In any event, all of the files are perfectly accessible now. But their filenames, and the name of the folder, are in green. If I create a new document and drag it into this folder, its name appears in green in Windows Explorer too.