Windws XP SP2 introduces a few new twists to TCP/IP in order to babysit users and "reduce the threat" of worms spreading fast without control. In one such attempt, the devs seem to have limited the number of possible TCP connection attempts per second to 10 (from unlimited in SP1). This argumentative feature can possibly affect server and P2P programs that need to open many outbound connections at the same time.
Rant: The forward thinking of Microsoft developers here is that you can only infect 10 new systems per second via TCP/IP ?!?... If you also consider that each of those infected computers will infect 10 others at the same rate:
second 1: 1+10 computers
second 2: 10+10*10 computers (110 new ones)
second 3: 10+100*10 computers ( 1110 new ones)
second 4: 10+1000*10 computers (11110 new ones)
....
all the way to 10*60 + 10^60 computers in a single minute (that's a number with 60 digits, or it would far exceed Earth's population). Even if we consider that 90% of those computers are unreachable/protected, one would still reach ALL of them within a minute.
To change or remove the limit, you can use the following program: (This patch is safe. Read the directions carefully :)
http://www.lvllord.de/
Here's another good program that follows the last: (Description: The TCP Optimizer is a free, easy Windows program that provides an intuitive interface for tuning and optimizing your Internet connection. There is no installation required, just download and run :)
http://www.speedguide.net/downloads.php