Computer hackers on a home laptop?

we have a dell notebook at my parents house that they would use to access the internet to do such things as send emails and check sites like facebook. microsoft however have rang my mother and informed her that they are hackers accessing our laptop and using our ip address and have informed us that they have left viruses and are the reason that the laptop often runs slow.this laptop has only ever been used to download music once and has nothing of value on it so im unsure of why anyone would want to access it.it has norton antivirus on it and im wondering if there's anything i can do to get rid of the viruses and block the hackers.microsoft want 250 euro for the pleasure of simply looking into it and im wondering if there's anything i can do other than take it to a computer shop..suggestions much appreciated!