Laptop keyboard: not working for passwords or cmd prompt; otherwise ok

Very strange keyboard behavior: for the last several months, I will occasionally resume from sleep and discover I cannot type my password at the login screen. I eventually figured out that I COULD use the On-Screen Keyboard that's available from the Ease of Access options on the login screen. Once I entered the password using the OSK, things would work normally for the most part. Sometimes when I'm doing something that requires a UAC prompt, I can't type the required password there either. Last week, I started a Command Prompt and discovered I couldn't type ANYTHING at the prompt. However, if I run Notepad or almost any other application, the keyboard works normally!!

Today, I tested further and found that Ctrl-key sequences work (e.g. pressing CTRL-M generates a carriage return at the CMD prompt but pressing the ENTER key does nothing). Also CTRL-ALT-DELETE is recognized.

This problem goes away temporarily when I restart Windows, but has always come back after some hours or sometimes days of use.

My first (paranoid) thought was that this might be some keylogger gone bad. Also I'm using Kaspersky Internet Security and wonder if it might be caused by some of their protection (anti-keylogger) software not working correctly (I'm also posting this problem on their forums).

Anyone ever seen anything like this?

Does it happen only when recovering from sleep or screen saver?
Do you have an external USB keyboard to try?

I think so far the problem always begins when I've resumed from sleep. However, it is not consistent: when I first restart windows, the problem is gone; it may not appear until I've resumed from sleep 10 times or it may appear the 1st time I resume from sleep. I've even had it GO AWAY on it's own after resuming from sleep! (but that is rare).

Should have mentioned I've tried USB-connected keyboard AND Bluetooth-connected numeric keypad and the problem exists there as well, so it's nothing to do with the physical keyboard.

Go to Control Panel\All Control Panel Items\Power Options and Change Plan settings - Change advanced Power settings - USB = disable