Page won't display in certain locations only

I have a website up and running but my client is having difficulties viewing the site (it does not display at all). Instead of displaying the site (which is coded in xhtml with an external CSS stylesheet) they get a dialogue box which prompts them to download a file (see image below):
http://www.ufp-squad.com/images/websiteerror1.jpg

The strange thing is that I can access the site from home and from my phone. I am not able to access it from work, nor can a friend of mine access from his place of employment (about 2 miles away), from his home (20 miles away) or from his church either. Everything looks fine from my end but I am at a loss as to why it cannot be rendered by any of the browsers (IE, FF, or Chrome). Any help would be appreciated. The url is http://www.stjohnsminier.org

Jack

instead of having my extensions read "xhtml" I would change them all to "html"

ok but I thought using strict xhtml I would not have to use .html extensions but instead use .xhtml and this still does not explain why it displays differently from different locations. :(

most likely has to do with what browser they are using.

Yes, lots of problems with the mark up.

IFRAMEs are not part of the XHTML 1.0 Strict DTD standard, but are OK in the XHTML 1.0 Transitional (I think! - I try not to use it.). This maybe a part of your problem. I loaded the page in FF 5 and the calender didn't look right. I had to re-load the page to get it right. - The top and bottom were cut off at first.

Your #content has height: 505px; but #content_right has height: 545px; It does show

Your footer wanders when text is inc. or dec.

I viewed this in FF with firebug, think you should also.

Can google calender be displayed without using iframes?