The other day I ran into a page here. It mentioned that XP SP2 and Vista have an updated TCP/IP stack where it was designed to reduce the risk of your PCs to spread out the worm in case it gets affected. Some people found a way to work around it by manually editing the TCPIP.sys file. You might need this if you have to write an application where it attempts to connect to multiple PCs concurrently such as web crawlers, peer-to-peer application, system monitor where the remote hosts cannot be expected to be up all the time.