Talk about a FAQ! Something very much like this appears at least once 
a week on the VNC mailing list:

"I'm at a work on my office PC. It's connected to a LAN that's behind
a firewall and a web proxy that I know nothing about. The IT guys
setup my web browser and email so that they work. But, I have a VNC 
server running at home, and when I start a VNC Viewer and type in my
VNC Server address, I get "connection failed" errors. How can I 
connect to it?"

The usual first response to this is more instructional than helpful:
if the IT guys at your workplace don't allow VNC Viewers to work behind
their firewall, they might have a really good reason. If you accomplish 
any "tricks" to bypass their security, you might just find yourself in
hot water with your employer. So ask permission first.

Once you have permission, the easiest way is do it like this: setup a
SSH tunnel between your work PC and your home PC. Many SSH clients are
HTTP-proxy aware, including the very popular PuTTY SSH client. Follow
these instructions for setting up an SSH tunnel with PuTTY, just be 
sure to add the correct "Proxy" information into the "Connection"
settings of PuTTY:

Using PuTTY's SSH on Windows

The above example presumes that the VNC Server is running on Linux, but
it of course works perfectly well if you're running on Windows too. Once 
you've got this setup correctly, your VNC Viewer traffic will pass right 
through your workplace firewall and proxy, just like your Web browser 
traffic does.