why wont my computer recognise my iphone

Ever since I got my iphone I cant get my laptop to recognise that my iphone is plugged into it. The laptop makes a noise as if a new device is being used but nothing happens and when I am in itunes it doesnt recognise i have an iphone plugged in. If anyone has any suggestions I would really appreciate them as I have had the phone for 12 months and still havent found a solution. Many thanks :-)