Windows automatically creates an IPSec policy for L2TP connections because L2TP doesn't encrypt data. However, in your situation that you need L2TP connection without the security of IPSec, you must disable IPSec on both the client and server:
On server side:
Open Services -- Click on Start > Programs > Administrative Tools > Services then disable IPSec service
In order to disable IPSec on Windows 7/Vista
1) Click Start, then in search box type: run ENTER
2) In the Run dialog type: regedit ENTER
3) In Registry Editor: Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters
4) On the Edit menu, click New>DWORD
5) As the name of the new key enter: ProhibitIpSec
6) Double click the key to edit the value.
7) In the Value data field, enter: 1
8) Quit Registry Editor, and then restart the computer.
Hope it could help