Learning Windows Admin would be a big plus, in my humble opinion. But, before we go down that road, what have you been doing with SQL Server? Are you a hardcore System DBA that rarely get's involved with applications and maybe has "average" knowledge of T-SQL? Are you an Application DBA that might not know very much about the administration of the system but knows the "black arts" of T-SQL? Are you a hybrid DBA with good experience in both of those DBA paths? Either way, how much of the security do you personally handle for your SQL Servers?