Hi Koala77,
Most HP computers if they are not to old have a system restore function. To start this, turn the computer off and disconnect any printers or external hard drives. You only need your mouse, keyboard and monitor to do this. Now hit the power button and turn it back on. As soon as you do this, start tapping the F11 key on your keyboard. This will start the built in system recovery. When the screen comes up, you need to click on "Advanced". If Advanced Options is selected, the system will perform a complete reformat and will destroy all user data. On Advanced Options, select the Windows System Restore option and click Next to begin the process. Then just follow the onscreen instructions.