By default Tableau Server will try to use port 80. But if for some reason that port is not available, it will select another port. If you bring up the 'Configure Tableau Server' applet you'll see the 'Gateway' section and this wills show which port Tableau Server is using to handle http requests.
If you're launching a browser on the same box that Tableau Server is running on you could just enter
The above assumes a default port of 80, but if Tableau Server is using a different value for the port other than the default, you'll want to specify the port address in the URL. For an instance I'm running it's actually using port 8000. In this case you'd want to enter:
On the one client PC having issue, you may want to try a different browser as well just to make sure it may not be the browser settings that could be causing an issue.