I'm working with a contractor in a different country.
We need some method to add his machine to the local network, using a point-to-point VPN.
Unfortunately, Hamachi, while very easy to use, creates stability problems as he is using Windows 7.
Whats the best method for us to use?
You've tagged your question "OpenVPN", which is exactly what I'd suggest using. I've run OpenVPN on Windows 7 x64 with no problems, beyond needing to choose "Disable Driver Signature Enforcement" when starting Windows. The OpenVPN GUI needs to be started elevated in order to alter the machine's routing table, as well.
OpenVPN is pretty easy to setup with static keys, and only marginally more difficult to setup with certificates.
If its at a static location, I really prefer hardware based point to point VPN. I like the Linksys (Now Cisco small business) RV042, they have served me well in the past, really reliably, and allows a fair bit of configuration.
We ended up setting up port forwarding on the router for the specific services we needed, and Hamachi 2 while we got the port forwarding working. Hamachi 2 is slow (seems to be limited to 8kbyte/sec), but it always works (unlike Hamachi 1, the subject of the original question).