Vista Recovery , can't see OS

I have run the auto repair on a Vista PC, I'm quite sure it has a corrupted MBR, but when I run the repair it tells me there is no OS. It identifies 2 partitions a 10GB which I assume is the recovery partition and a 260GB which is the OS.
When I try to browse the OS it tells me that it is not mountable.
Also when I try to open the partition I get "The enclosing drive for the volume is locked"

Any suggestions?

Thanks in anticipation
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