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If there is any posability of NAT being between the road-warier and the VPN server it is very important to set the value
AssumeUDPEncapsulationContextOnSendRule
to 2 in registry subkey
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPsec

See http://www.jacco2.dds.nl/networking/freeswan-l2tp.html
and http://support.microsoft.com/default.aspx/kb/885407/

VPN:IPsec:Connect Windows XP to Linux L2TP IPsec VPN server is mentioned on: VPN:IPsec | Windows