Start by reading though the documentation at http://www.realvnc.com/support/getting-started.html and http://www.realvnc.com/support/faq.html.

The steps you'll need to carry out are:

1. Install the VNC server on the Windows PCs - I don't think RealVNC is working properly on Windows 7 yet though, so you may want to use UltraVNC (http://www.uvnc.com/) instead.
2. If you're wanting to access the PCs from outside your local network, you'll need to set up port forwarding on your router. If you read the FAQ, there's a link there to documentation on setting this up. If you don't have a static IP address then you'll probably also want to set up dynamic DNS - there's quite a few providers for this, and a lot of routers have built-in support.
3. Install the VNC viewer on the N800.