Winxp will use ipsec to encrypt l2tp by default, which can be canceled by modifying the registry. Win7 doesn't seem to have this problem.
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters
Add the following registry value to this key:
Value Name: ProhibitIpSec
Data Type: REG_DWORD
Value: 1
https://support.microsoft.com/en-us/help/258261/disabling-ipsec-policy-used-with-l2tp