By default Windows XP allows a maximum of 10 half open connections at a time, this limit is implemented as a security feature to slow-down spreading of internet-worms in-case the user machine gets infected, if your network use is crossing this limit your computers event-log will show the following Event:

EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts.

This limit can slow down your internet access specially Peer-2-Peer applications like BitTorrent which require connecting to several peers for optimal performance, to increase this limit for speedy torrent downloading you need to patch the Windows TCP/IP system driver file, If your have already patched your TCPIP.sys file and updated your computer to Windows XP Service Pack 3, you should re-patch it using the free TCP/IP patcher for Windows XP.