PDF form is unsaveable in IE for some users and not others

I have a pdf form in internet explorer that for the majority of users works a treat. However there are some who get the message:

"Please fill out the following form. You cannot save data typed into this form.

Please print your completed form if you would like a copy for your records."

I've checked that these people are on Adobe Acrobat Std, rather than Reader, and I've checked the pdf file association is correct.

Has any ideas as to why this doesn't work? Is there an obscure file association for pdf forms in IE?

Thanks in advance.