These 'ghost' files are prefixed '._' yes? If so, that'll be ye olde Mac resource fork file - which a PC wouldn't understand. Your PC user probably sees a blank icon on the 'proper' file? Suggest zipping the file first before attaching to the email. (In Tiger, right click file and "create archive of xxxxx").
We occasionally get these, the 'proper' file can be opened by manually adding the . extension (ie. .xls or .doc) - assuming a success, the 'ghost' file can be safely thrown away.
HTH