Repair Winsock in Windows XP SP2.
Some spywares like NewDotNet hijack the Winsock layer of Windows. When these spywares are incompletely or incorrectly removed, Winsock layers are not restored to their original state. Due to this, Internet connection is not possible. In Internet Explorer, you get "Page can not be displayed" error, if Winsock layers are corrupt.
To restore the Winsock layers, there are third party tools like WinSockXPFix and LSPFix. But in Service Pack 2, a new command has been introduced, using this command, Winsock layers can be very easily restored. All you have to do is, type netsh winsock reset in Command Prompt and press ENTER key. After this, restart the computer for the changes to take effect. (Command Prompt is present in Start > All Programs > Accessories)
For more information on netsh command and it's switches, check this page.
For non Service Pack 2 systems, WinsockXPFix or LSPFix can be used to restore the Winsock layer.