With the release of Service Pack 2 (SP2) for Microsoft Windows XP, the SP2 has introduced a feature that limit concurrent TCP connection attempts that are possible to 10 per seconds. In Service Pack 1 or without Service Pack, there is no limit on concurrent TCP connection attemps.
This feature is introduced in order to reduce the threat of computer worms spreading too fast without control. With the limit, only a maximum of 10 connection attemps per second are allowed. This may have affects applications, servers and p2p programs that attemp to open many outbound connections at the same time.
To know if you’re hitting concurrent TCP connection attempts limit, use the Event Viewer. Under “System”, look for TCP/IP Warnings saying: “TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts”. It has the Event ID of 4226.
To change or remove the limit imposed on connection attempts in Windows XP SP2, you can use the following program that works on every language of Windows XP:
Event ID 4226 Patcher version 2.23d
You might want to back up the tcpip.sys, although the program has the ability to restore tcpip.sys to original state.
Update: TCP/IP simultaneous connection limit patch for Windows Vista