For starters do you have a reverse dns zone and can the SIM server resolve the discovered ip adresses?
Can you ping the IP-adres you scan?
Sim pings IP-adresses if it can ping them it will try a number of protocols like SNMP, HTTP, WBEM and SSH. With WBEM wich is the best for Windows you only need an account that has local Admin rights. So Lets try that first. The first thing you can post id the log for the identification of a Windows server.