Try RealVNC instead as this is the most widely used and the basic version is free. You'll only need the basic version anyway as the enterprise one just includes more encryption. click here and see if it works any better. Just install the server on your home PC and you can access it through a web browser at work by typing your public IP in and put the port number at the end (which is 5800 for web browser). If you want to try again with UltraVNC you might also need to put the port number in for that, which by default is 5900 for the software viewer and 5800 for the web viewer. So put in yourpublicip:5900 and see if it works then.