Problem using internet connection sharing

I have a problem using internet connection sharing.
I have one laptop with Windows XP connected to my company LAN and accessing internet successfully via the company proxy server.
I have a second laptop with Windows 7. I want to allow this second laptop to access the internet through the Windows XP laptop connection.
I connected the two laptops via a computer-to-computer wireless connection. The connection is working fine and I can copy files between the two computers.
I enabled internet connection sharing on the Windows XP laptop Local Area Connection. Its wireless connection has IP address The IP address of The Windows 7 Laptop is (automatically acquired).
Internet Options on the Windows 7 Laptop is set to "Never dial a connection", not using a proxy, I tried with and without "Automatically detect settings".
Now, the network icon in the Windows 7 system tray indicates that the wireless connection HAS internet access. Also, in the Network and Sharing Center, the wireless connection access type is "internet". So, Windows 7 knows that this wireless network has internet access. However, both internet explorer and fire fox cannot open any web sites. Exactly as if there were no internet connection!. They give "Internet Explorer cannot display the webpage" and "Firefox can't establish a connection to the server at
." respectively.
Internet is working properly on my Windows XP laptop. Also, when I connect the Windows 7 laptop to the LAN using a UTP cable or wireless connection, internet works properly on it. I just cannot access the internet via internet connection sharing although Windows 7 feels there is internet access on the connection.
I tried to disable all network connections other than the wireless connection on the Windows 7 laptop, log off and log on, restart, but still have the problem.
On my Windows XP laptop, in the LAN connection properties, Advanced tab, I clicked settings and tried checking the services HTTP, HTTPS, FTP, DNS but also with no avail.
I would appreciate if any one can help me fix this problem.
Thanks in advance.
Regards M.Afifi

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