Now if your new to BitTorrent, you might be new to downloading. If so you should know that Microsoft introducted a limit to stop many TCP connection attempts in XP SP2. This well-intentioned modification came as an attempt by Microsoft to limit the speed at which malicious worms (such as the infamous ‘Blaster’) could spread.
However, for software that relies on many connections such as bittorrent applications, this limit has the potential to cause slowdowns in certain situations. So what you want to do is change the half-open limit to a higher number.
For Windows XP SP2: http://www.lvllord.de/?lang=en&url=downloads
unzip it to your desktop and then run the .exe file. Follow the instructions and then change your limit to 75 or higher. Make sure you restart your computer immediately after it is finished--like it tells you.
For Windows Vista: http://www.yaronmaor.net/repair.htm
choose the right fix depending on your version of your tcpip.sys file.
There, now you will certainly increase your download speed, and possibly your upload speed as well.