Sounds like you have a Malware infection in that users account.
Have you tried booting up under an external drive and run both Disk Utility to check the drives permissions (do the disk repair as well) as well as run a good antivirus/malware program.
If you still can't get the account to work you'll need to backup your data (after booting up under the external drive) then deleting the user account and creating a new one. I would recommend you also create a second admin level account to have for the next time. If you had it you could have tried it.