A keylogger running in the background could record all your keystrokes and send them away if your PC is connected to the internet. There is small program AKLT (AntiKeyLoggerTester) which can be used to check the protection of your PC against keylogger. The homepage of this tool is offline, as it was written 2010, but it can be download here:
http://www.snapfiles.com/get/antikeyloggertester.html
You can check that AKLT.exe will most likely pass your antivirus-software:
https://www.virustotal.com/de/file/fdbb91d383615b64e6abaf327d1ec5567f3289657197faacbc57ced90f975b7f/analysis/
If you are want to check if AKLT.exe is legit, you can take look at wilderssecurity: https://www.wilderssecurity.com/threads/aklt-exe-firewall-leaktester.239170/
Start AKLT.exe, most likely your antivirus will do nothing:
Now press the GetKeyState and write something somewhere else (AKLT is out of focus).
If AKLT is now looking like this:
then your system is unproteced against keylogger. Be carefully when entering the secret key phrase into your wallet...
Comodo Free Firewall
I'm using Comodo Free Firewall to protect my PC. The standard setup is not sufficient. I'm using the setup from cruelsister1, which is shown here:
With this setup, all unkown programms and scripts are running automatically inside a sandbox.
This is shown by a green box.
After pressing GetKeyState, AKLT is not able to record my key strokes:
How do you protect your Windows PC against keylogger?