You should be able to sort this out via the Control panel under User Accounts.
I do assume though that your girlfriend has clicked the NTL icon (which I assume you have) and then input the NTL User Name and Password again for her that you had to do for yourself. You can't just click the Internet Explorer icon from the desktop in the same way that you could with a dial-up modem.
I know that I have made a couple of assumptions here which may not be of help, I use Pipex myself and so am unfamiliar with NTL. If they are not helpfull or you need help with the Control Panel then post back.