I managed to how to bypass this problem, I did not find out what proxy is used here, but I used the following steps to solve the problem:
"The problem is the autentificación NTLM of Windows7. It is necessary to create the following key in the registry to solve it (I'm using Squid Version 3.0.STABLE8 in Debian Lenny):
1. In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
If it doesn’t exist, create a DWORD value named LmCompatibilityLevel and set the value to 1 to use LM NTLM and NTLMv2 if is negociated, this is
Also it works establishing the value to 0, and 3 though for more safety the value using 3 though with old operating systems it will not work on having used obligatorily NTLMv2.
To follow the link for more information: http://technet.microsoft.com/es-es/magazine/2006.08.securitywatch(en-us).aspx"
This worked well, now I'm able to connect in internet through proxy server.