Are you using SP2?
There are supposedly some bittorrent issues caused by it because it limits the number of TCP connections to 10.
From another site:
"After installing SP2, Windows is limited to initialising only 10 TCP connection attempts at a time. This does not mean you can only have 10 connections, it means you can only have 10 connection attempts occuring at once, for example, if your PC tries to initialise 20 TCP connections, the first 10 will succeed and the second 10 will be queued. "So what?" you might say. Well, if you use BitTorrent (Azureus etc.) to download files then this may slow up your speeds considerably. The first thing you should do is use BitTorrent as you would normally for a few days. Then do this:
Go into the Administration Tools section of Control Panel and check out the Event Viewer. Look in the System section and see if you have any entries like this:
Source: Tcpip Event ID: 4226
TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts.
For more information, see Help and Support Center at http://go.microsof t.com/fwlink/events.asp.
One or two of those is acceptable, but if you have a huge list of them, you have a problem with Windows stifling your connection. The best way to fix this is to use Azureus as your BitTorrent client and make a simple configuration change in the program:
View - Configuration - Transfer - Check the box next to "Slowly connect new peer connections etc. etc." - Hit the Save button. Restart Azureus.
This should cure your problem. If it doesn't, you may need to look at applying a patch that increases the number of TCP connection attempts allowed."