I'd contact Virgin asap. I don't like the sound of this as it may cost you a fair bit of money. I know you use a mobile dongle for internet so it may have been hacked. I don't know anything about mobile internet so contact your supplier now, just in case.
I would still not be happy. You now use dial-up but when using the dongle you get Orange roaming? I strongly suggest that you follow this up again as something is wrong. Run your security software and if you can change your password when using the dongle once you login. Use a lot of characters as well as letters and digits.