When you installed or set up Windows you set it up with US keyboard settings and you have UK keyboard.
As you don't say what version or Windows you have here is some generic advice. You need to System Settings an change the keyboard to UK. As you have set the keyboard to US you may have set the Regional Setting to that as well and so I'd check that.
BTW, you haven't changed your modem, you've changed your System Unit. A modem is something completely different. You may as well get it right and then you won't confuse people.
T
August 2009