Fix from here: http://forums.whirlpool.net.au/archive/1978519#r35738525 via http://communities.vmware.com/thread/419232
- Using the 2012 box you're trying to install vCenter 5.1 on, go to HKLM\System\CurrentControlSet\Services\vpxd
- Open the DependOnService key and remove ProtectedStorage from the list
- Ensure the vCenter Server and WebMgmt services are using domain logons
- Start the vCenter Server service - will take a bit, but will work
- Re-run JUST the vCenter install for it to complete successfully
I spent several head-scratching hours trying to figure this out, now working like a charm.