Ever try to open more than 2 downloads at the same time in IE but had it not open the third one until one of the other two had finished downloading. Do you find this annoying as hell?  Well, today is your day <g>.

from http://support.microsoft.com/support/kb/articles/q183/1/10.asp

SUMMARY

WinInet will limit the number of simultaneous connections that it will make to a single HTTP server. If you exceed this limit the requests will block until one of the current connections has completed. This is by design and is in agreement with the HTTP specification and industry standards.

MORE INFORMATION

WinInet will limit connections to a single HTTP 1.0 server to four simultaneous connections. Connections to a single HTTP 1.1 server will be limited to two simultaneous connections. The HTTP 1.1 specification (RFC2068) mandates the two connection limit while the four connection limit for HTTP 1.0 is a self-imposed restriction which coincides with the standard used by a number of popular Web browsers.

The only evidence of this limitation to your application will be that calls like HttpSendRequest and InternetOpenURL will simply appear to take longer to complete as they wait for previous connections to be freed up before their requests are sent.

You can configure WinInet to exceed this limit by creating and setting the registry entries below

NOTE: By changing these settings you are causing WinInet to break the HTTP protocol specification for your application and any other WinInet applications running on your machine, including Internet Explorer. You should only do this if absolutely necessary and then you should avoid doing standard Web browsing while these settings are in effect:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings

MaxConnectionsPerServer REG_DWORD (Default 2)
Sets the number of simultaneous requests to a single HTTP 1.1 Server

MaxConnectionsPer1_0Server REG_DWORD (Default 4)
Sets the number of simultaneous requests to a single HTTP 1.0 Server