I don't think that this works 'out of the box' .. Simplest way would be PPTP or L2TP connection, but as you asked for site to site, I would suggest to install a firewall or vpn-product on the windows machine that supports your requirements.
The Windows machine will have to be a Win2K or Win2K3 server, with the routing package and Microsoft ISA (Internet Security and Access) installed on it. Then you can set up an IPsec site-to site tunnel between an Astaro box and a Windows machine. Doing it this way also means that you'll end up with a Windows machine exposed to the outside world at one of your locations.
In terms of software licensing cost, consuming a Windows server license and an ISA license in order to implement this, it will end up costing you more than if you simply got yourself another Astaro license for the other site.
By the way, Microsoft's recommendation for using Windows server with ISA installed as a firewall, is that you run two such machines, one behind the other, with the DMZ net inbetween them. This doubles both the hardware and the software cost.
Your best bet is to have an Astaro box at each site with a VPN tunnel inbetween them. Putting a Windows firewall in one end will give you a solution that provides less security at a much higher cost.
