Windows normally limits the number of simultaneous connections made to a single web server. This behavior can be seen in Internet Explorer when downloading multiple files from a web site and only a certain number will be active at any one time.  
 
Windows 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.  
 
Open your registry and find the key below.  
 
Create two new values, or modify the existing values, called 'MaxConnectionsPerServer' and 'MaxConnectionsPer1_0Server'. Change the values to equal the number of simultaneous requests allowed to a single HTTP server, the default values are 2 and 4 respectively.