Because you have no means of closing the pop-up FBI alert message, you will have to boot your machine into Safe Mode with Networking, which will give you access to only the basic files and drivers. Safe Mode with Networking allows you to connect to the Internet, which you will need access to in order to download anti-malware tools that will help you remove this virus.
Power up your computer and press F8 just before the Windows splash screen appears. This will prompt you to the Advanced Boot Options screen. Using your arrow keys on your keyboard, highlight Safe Mode with Networking and press Enter. While in Safe Mode, you will notice that your desktop background is replaced with a solid black color.