Webmail Giving Problems

My partner and I use the same webmail service. I can log on to my account but he can't log on to his. We've phoned BT Yahoo (in the UK) who provide the webmail service. They asked permission to log into his account and said they could log in. We still can't log in. They don't know why. Can you help?

The PC has Windows XP Home Edition with SP3 and according to Windows Update everything's up to date.

We access the web using Firefox using the current version.

Before trying last log in cleaned the PC using both the Disk Cleanup utility and CCleaner. I can still log in but my partner can't. Really confused by this.