If your ISP provides a fixed IP, then skip the next step.
Go to
http://dyndns.org and sign up an account + grab a Dynamic DNS account a , download and install DeeEnEs :
http://www.palacio-cristal.com/products/DeeEnEs - set up the software so with the Dynamic DNS.
Ensure the IP address on your home PC doesn't change, assign it a fixed IP or ensure it's the only box on the network.
Set up your firewall to pass a randomly chosen port (somewhere between 10,000 and 65,000 will be fine!) to the fixed IP of your home PC.
Install VNC, configure a good strong password, bung some spaces in and random letters and numbers ... NO REAL WORDS and at least 10 characters in length.
Configure VNC to listen on the port your chose above.
Point your work PC to connect to the Dynamic DNS address, using the port chosen above and the password above.
Points to note, you must change the port otherwise you'll have every Tom, Dick and Harry trying to crack your VNC Password, being on a random port gives you a lot more protection through obscurity.
Consider encrypting the network traffic using a tunnel such as Zebedee:
http://www.winton.org.uk/zebedee