Running Heroes II on both a Windows 7 and a Vista x64 machine, wanted to do LAN but of course the only option available is NETBIOS, upon clicking the icon and selecting either Guest or Host it closes DOSbox immediately.

Are you running from the host and client shortcuts?

I've had a little look around and I tried going start -> gog -Heroes II -> network -> Host and the game still closes after in game I click "Netbios - Host"
Could someone detail me what I should be doing to try to get this running between the two PCs in the first post, if at all possible?

Many thanks.

You need HoMM II Gold for Windows game version to host/join multiplayer games. GoG version of the game is not a Gold version at all (it made from HoMM II Compendium pre-Gold version of the game released in 1997, it was a last DOS v2.1 release of HoMM II). Original HoMM II Gold (for Windows) released October 1998 by New World Computing/3DO.

but it says multiplayer when u look at the game...
it clearly says "modes: Single-player, Multi-player"
so why say multi-player when it's not even possible?