How to allow the local Administrator to have access to SQL Server?
If Jason's suggestion doesn't work, restart the SQL Instance in single user mode and everyone in the local admin group should now have sysadmin rights so you can get into the system and fix the permissions.
There should be a local group created called SQLServerMSSQLUser$$SQLEXPRESS on the machine. Add the user to that, restart SQL service and then log off/back on and you should have the needed access. Then you can add the administrators group to the instance explicitly.