downloaded & installed space marine with map packs & power sword dlc (75% discount weekend @ steam store 1st jul 2012)
clicked to play game - "steam error: this game is currently unavailable"
via control panel > add/remove programs > removed all instances of Microsoft Visual C++ Redistributable
used c cleaner to tidy registry errors http://www.piriform.com/CCLEANER
(found both visual C++ 2005 and 2010 incompatible with my copy of space marine)
downloaded & installed: Microsoft Visual C++ 2008 Redistributable Package (x86 version my case)
steam running space marine, huzzah.
incompatible ms visual c++ 2005 redist, bundled with steam store spacemarine.
win 32 xp sp3
intel dual core 2.93 ghz
2GB ddr3 ram
nvidia gt240 512MB ram
80 gig free hdd space