Windows Live OnceCare will block VPN ports

If you tried VPN connection at Windows XP or Windows Vista, but failed with no reason given, that probably because of the FireWall settings. If you use Windows Live OnceCare, that might be the cause of the problem. Open OneCare, click "Change OneCare Settings", click "FireWall", click "Advanced Settings",click "Ports and Protocols", tick the box "Virtual Private Network (IPsec of PPTP)". Done. The problem should be solved!

If the VPN connection still failed, try to look at other FireWall or anti-virus application. Usually Lower down the FireWall level or disable it would do, but be careful.

If problem still there, it might be Visa TCP Window Scaling problem, try this http://support.microsoft.com/kb/934430