Before attempting this, please make sure that Service Pack 5 or better is installed.

Navigate the the location below in your registry and make the corresponding change.

Create or edit the existing DWORD value named 'DisableIPSourceRouting'.  Set the value to either '1' to disable source routing, or '0' to enable source routing. 

Restart Windows. 

Key:[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
Value Name: DisableIPSourceRouting
Data Type: REG_DWORD
Data:'0' or '1'
More info: http://support.microsoft.com/support/kb/articles/q217/3/36.asp