EventID 4226: TCP/IP has reached the security limit
Windows XP with Service Pack 2 and Windows Vista have both a TCP/IP connection limit that limits the half-open connections of the system. If that limit is reached a new entry in the Event Viewer is created stating "EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts.".
It is not absolutely clear to me why Microsoft did set the limitation, some possible reasons could be to prevent worms and other malicious scripts from spreading to fast or to limit filesharing. Users will most likely notice that something is wrong when running P2P clients, slow downloads and timeouts are indicators that the limit is set.
The easiest way to check if the limit is responsible for slow downloads or other issues related to downloading files, is to check the Windows Event Viewer for these events.
To open the Event Viewer click on the start button of the operating system and enter Event Viewer in the search and run box there. Select the program from the list and wait until it has loaded. Now search for the eventID mentioned above and see if you get hits.
Thankfully a solution exists that can patch the file tcpip.sys and remove the security limit. To do that you need to download the file Vista TCP Patch, open an elevated command prompt with administrator rights and enter the following command.
VistaTcpPatch /n X with X being the maximum amount of half-open connections allowed on that system.
The computer needs to be restarted afterwards. Some users reported that they needed to execute Vista TCP Patch from the \Windows\System32\ folder to make it work.
Windows XP users can download and run the software EventID 4226 Patcher Version 2.23d instead which basically does the same thing but sets the limit to 50 half-open connections.
Vista TCP Patch (for Windows Vista)
EventID 4226 Patcher Version 2.23d (for Windows XP)