The ports which are open are port on wich the OS listens on. Thats how NMap is able to determine what type of OS the victim is using so quickly as it checks for what listening ports there are. To stop Netbios you should go into your internet options (Start -> Settings -> Control Panel -> Modem [I think]). From there you can disable Netbios.
I also suggest that you use a good firewall such as Sygate and add a rule to it to block everything on the ports 135-139 and make sure that it only allows the programs you trust the most to access the internet. Everything else has no reason to go there.