403 Forbidden: The ISA server denied the specified url

it’s a funny problem actually and i don’t know what to do anymore.
i have three computers, all in one office, and they cannot open our web page. they get the above mentioned error. the funny thing is that everyone else in the company can open the page, even people who are in the same user group on the ISA server as these three. i tried to change the group and i put them in the power user group, but they still couldn’t open the page. the error changed this time and it’s stating the host is unreachable. i forgot to mention that they always could open any other page except our web page. the only problem is with our web page, and the problem appears only on three computers. has anyone ideas? tnx