Like Windows XP SP2, Windows Vista too does limit the maximum number of half-open TCP/IP connections to 5 and 10 for Vista Home and Ultimate Editions repectively.

The following event is logged when the maximum number of connections has been breached.

EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts

There is an excellent archive at klitetools that contains a hacked version of the tcpip.sys drivers for Windows Vista 32-bit and 64-bit Edition and a batch file that automatically:

a) takes ownership of the tcpip.sys driver from c:\windows\system32\drivers
b) gives the currently logged on user account full owner permissions
c) adds a key to the registry under
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\]
d) renames the original tcpip.sys to tcpip.original and replaces it with the hacked tcpip.sys driver

To install the patch,

1) Download and extract the contents of the package to a folder on your computer from here

2) Run the batch file contained in the package depending on your Windows Vista version- Patch32.bat for X86 version and Patch64.bat for X64 version.

3) Browse to [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\] in the registry using “regedit” or Windows Powershell
and change the value of “TcpNumConnections”to anything you want in decimal.

To do this using Powershell,type the following in the command prompt:

PS C:\> Set-ItemProperty -path “HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\” -name “TcpNumConnections” -value xyz

Note: If you get an error about permission or access denied, temporarily turn off User Account control from the Control Panel

4) Reboot the computer