Do your other Internet related applications "not respond" or do they simply fail to connect to the Internet? Given your seeming to indicate you have no idea what could be causing an issue, have you tried using System Restore to restore to a time before this issue arose (in Vista, type "System Restore" in the search box in the Start menu, then choose the "Recommended restore" (unless the given date and time are after when you found yourself unable to access the Internet, at which point you should choose an earlier restore point))? This is probably the easiest way you could attempt to fix the issue. It's difficult to make a guess as to the issue without your having idea what might be the cause, so, as stated earlier, it won't hurt to ensure you have sufficient protection for your computer, because that may alleviate whatever is happening now and prevent it from recurring.
In regard to security (assuming you have authority over the computer; you mentioned your mother, so I don't know), I would suggest you try either Avast Antivirus or MSE (both of which are free). Also, get Malwarebytes. I wouldn't bother getting a firewall aside from Windows Defender (the default firewall, which should be enabled) or MSE (if you choose it over Avast). If you need something more advanced to restrict the connections programs on your computer are making, then you have something on your computer that shouldn't be there (which means you have a gaping hole somewhere else, and have therefore have already lost). DEP should be enabled by default. As _____ said, definitely make sure your programs are up to date (and automatic updates enabled).