Windows contacts can't open my attachments

I'm using Mail in OS 10.5 and an increasing number of my (sad) Windows contacts are not able to open attachments I send them. They can see a 'picture' of the image in the body of the mail, but 'there's no attachment'. Then they reply to the mail… and I get their reply with the attachment present and all opening fine at my end. Also occasionally I send .xls files and .doc files and they open at their end and there's no visible data in the file (although the attachment properites show the file size that is as it was when it left me). I suspect these two issues are related.
Any ideas?

Just a WAG here - but it may very well be a security issue on their end. They may have IE set to high security or not to open attachments. Don't forget, Outlook Express (Windows mail program) uses internet security settings from IE.

Regards.