Is that publicvpn.com?
I had a quick look. It seems to support MS PPTP VPN (among others), it even claims to work (unofficially) with PalmOS, which should mean quite normal MS PPTP. If so, it should in principle be possible to get it working on the N800. You will have to install some additional stuff on your N800, which may or may not be outside of what you feel comfortable with doing. The ITTF member Fanoush has what you need at this link: http://fanoush.wz.cz/maemo/#pptp
Disclaimer: I have not tried this myself, although I've planned to for a long time - I need pptp occasionally.