Google gives up a number of possible solutions:
http://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=%22Unmountable+Boot+Volume%22
I just built a new system. It has a AMD 3000+ cpu,
120gig Western Digital Harddrive. Dvd Burner. When
installing Windows XP Prof. the partion goes well
and when rebooting shows Window screen then the blue screen which says "Unmountable Boot Volume". It mentions rebooting machine. Possible Hardware or software issue. Anybody have this problem? I have built several machines and never seen this problem I even trying reloading Windows same thing. Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic