Trend Micro Ransomware Screen Unlocker 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.)
To use the Unlocker you must be able to boot your PC to Safe Mode with networking. (If you can't do this, check out Trend Micro Ransomware Screen Unlocker for USB instead.)
If you can reach Safe Mode with networking, install Screen Unlocker from the download. Trend Micro warns that XP users should "make sure to uncheck Protect my computer and data from unauthorized program activity before running the tool".
Once you're set up, reboot to the normal lock screen message, and you should be able to launch Trend Micro Ransomware Screen Unlocker Tool by pressing Left CTRL + ALT + T + I.
A very simple console appears. Click Scan to look for the threats, "Fix Now" to remove anything found, click Reboot and your system should be back to normal.
Trend Micro Ransomware Screen Unlocker may be able to restore access to your PC, but only if you can use Safe Mode. If you can't, check out the bootable USB version instead.