SharePoint User Code Host starts then stops with ULS errors:
Performance Counter OS (pdh) call failed with error code PDH_CSTATUS_NO_MACHINE.
- Process perf counters are disabled. Sandboxed code monitoring cannot continue
Log Name: System
Source: Service Control Manager
Date: 7/30/2013 12:00:10 PM
Event ID: 7034
Task Category: None
The SharePoint User Code Host service terminated unexpectedly. It has done this 18 time(s).
The solution is described in http://support.microsoft.com/kb/983081:
add windows service account of SharePoint User Code Host to local server groups 'Performance Monitor Users' and 'Performance Log Users'