Rodmiester,
An important question to clarify is if they can see the attachments on your emails but can't then open them or if the attachments don't seem to be there when they receive your email?
If they can see your attachments but can't open them, then 1) make sure that they are a common format such as .JPG; and 2) make sure that your file names all have a file extension at the end (such as MyPicture.jpg). Although your Mac does fine with many files that don't have the ".jpg" at the end, their Windows PC will have no idea how to open them unless they manually add the correct file extension.
If they aren't seeing your attachments at all, then my suggestion is to kindly suggest to them that Outlook Express is very outdated email software and it would be advantageous for them to upgrade to a more modern email application. Mozilla Thunderbird
http://www.mozillamessaging.com/en-US/thunderbird/ is a free download and is certainly a great replacement for their aging Outlook Express (OE). It's possible that you are sending them "AppleDouble" encoded attachments that OE doesn't understand. You could also try changing your preferences in Apple Mail or Entourage to send attachments using "MIME" encoding. That should solve the issue if it's encoding.
Good Luck.
