My ISP uses NAT'd ip's meaning I cannot connect to it (using PCA, VNC etc) while I'm at work, unless I leave it connected to my works VPN (which I don't want to do)

Does anyone know of any way that I can make my home pc connect to the vpn when I tell it to (for example with an email or instant message) so I can connect to it, or some other software that will make my pc initiate the connection, and then allow me to controll it remotely?

or any other ways around the NAT?

Cheers for any help