I am trying to play Dark Messiah on windows vista (64 bit). My system absolutely trashes the recommended requirements so that's all good.

My problem is that I've installed the game and updated it via steam, but when I try to launch it, I get an error that says:

"You do not have enough disk space to run this game. Please free up some disk space and then try again."

I have 218GB of free space on my hardrive, so I cannot see how this makes sense.

Thanks

Possibly, if you have different partitions, Steam means the free space on the partition Steam (including the game) is installed onto isn't enough.
If you have all on one partition, possibly:
you have hybernation enabled which copies your entire drive into a system page file.
The page file will be extremely large, by default double the physical RAM. Add to that the hybernation file hyberfil.sys, as large as your complete installation.

But apart from this, Steam is insane by nature: in the game's properties it shows the installation info of the multiplayer game instead of the singleplayer game.

Anyway, always verify integrity and defragment via Steam > My Games > properties of the game.

And could you post your dxdiag.