There is a bug in Windows XP Service Pack 2 relating to a loss of network connectivity for computers that use Microsoft's L2TP-based (VPN) client. You may also notice that your computer gets stuck acquiring a network address.
To fix this problem you need to download and install the Microsoft patch (KB884020). You can download the fix here. After installing the fix you should reboot your computer.
If you still experience the same problem after the fix you should check your network for problems including faulty network cables or network cards. You should also check to make sure your DHCP device is functioning properly and that your computer is set to receive an IP address automatically.