Cannot send attachments

My daughter is having a weird problem with email. She has Yahoo email and
although she can receive emails with their attachments just fine,
attachments she sends out from her computer do not get sent out.

It is not a matter of not knowing how: If she sends emails with attachments
using her account - BUT from another PC - the attachment is sent out
correctly, but from her own notebook, even though the attachments get
attached they do not go out.

For instance if she sends out an email with attachments to herself, she
receives the email minus the attachment even though the attachment is marked
as being part of the email. If she sends herself the same email from
another computer, using her same account, she receives the email with the
attachments.

She is stumped. What could be causing this problem? Her notebook runs on XP
Home.

Thanks,

Jeff