Trend Micro Ransomware Screen Unlocker for USB is a free tool for removing "lock screen" ransomware, malware which pops up when your computer starts and won't allow you to access the rest of your PC.
(To be clear, the program looks for this access-blocking malware only. It can't help you deal with file encrypting-type malware, and it's not able to decrypt your data.)
Launch the package and it prompts you to choose a USB key. This will then be overwritten with Trend Micro's bootable recovery environment.
Start your computer from the key and a very simple console appears. Click Scan to look for the threats, "Fix Now" to remove anything found, remove the USB key, click Reboot and your system should be back to normal.
There's nothing new about bootable antivirus rescue environments, and we're unsure whether this one is any better than the others. Still, it's certainly easier to use than most, and if you're having trouble accessing your PC then maybe it can help.