You should check your pc you might have a rootkit installed, maybe your web browser was hijacked visiting a webpage. If it is so, you are not free of danger yet. You should change all your passwords too, OS and mail included, but not before scanning your pc.

Better to use firefox with adblock and noscript activated and activate java only if you need it and only when you know what you are doing.

Use one offline antivirus software to check it, maybe something like hirens boot usb or cd. Also try actualizing your OS too.

Or you might just format your pc and reinstall from scratch.