Error 797 relates to a dial-up connection, which only a very small minority of people use these days. If you happen to be one of them, see here:
https://support.microsoft.com/en-us/kb/317222
Otherwise you probably need to stop Windows XP from trying to use a dial-up connection when one doesn't exist. Open Internet Explorer (even if it's not your usual web browser). Go to Tools > Internet Options. Click on the 'Connections tab' and select 'Never Dial a Connection'. Click 'OK'.