Microsoft has set in the past a default limit of 10 max half-open TPC connections in Windows, in order to protect users from malicious bots, spyware etc.
The problem is that p2p applications (e.g. torrent, eMule) require more half-open connections to function properly.
By patching the tcpip.sys file, Windows XP users managed to increase the default limit.
However the latest Windows update (January 08, 2008), KB 941644, resets the default limit to 10 connections.
Therefore, if low performance on p2p applications is noticed, the patch has to be re-applied!