Hi Sarah,
you have probably tried this link already but there is loads of help on o2 broadband there
Help
if that doesn't work i would first try and disable the wireless security on the o2 router which you can do by Opening your browser entering '192.168.1.254' from there you will be able to access the wireless settings.
If you still cant get access with the security disabled try and right click your wireless icon in the system tray and chose "repair" if that still doesn't work then you connection may still be set up with details of your old router in which case you would have to reset the tcp ip settings for your connection to the default see here for details of how it should look
Checking TCP/IP settings
hope this is of some help
