Windows 7 "This content cannot be displayed in a frame"

I get reports of people receiving the above error when accessing my website Link Removed on Windows 7 using Internet Explorer but works ok on other browsers. I don't get the error myself so I can't directly investigate it. I tried Googling the issue but all the results appear to relate specific issues / upgrades and not related to ordinary websites.
Can anybody advise please? I believe the users could add it as a trusted site under IE options but I don't understand why this should be necessary, the error doesn't appear to be giving a content warning.
Thanks.