Seems like you're machine is using US layout.
Steps for Windows XP:
Go to control panel>Regional and Language Options. Set both drop downs to English (Ireland). Click the Languages tab. In this window add English (Ireland) if it's not there already and remove any other option (may need to reboot to take effect), Press OK here.
Click Advanced, make sure English (Ireland) is selected, press OK.
If you have had to make any changes here, try a reboot and retest that key.
If when typing the @ key nothing happens then it may be that the keyboard is not working correctly.