Most of my friends don't have pets, but those who have might recognize the situation of returning to your computer to find your darling cat have posted "+666" on facebook, or watching a VLC movie when cat pushes a few keyboard commands and speeds the sound up by 200%. Easy enough fixes, but also annoying, and windows has no built-in keyboard lock. So, I found a tiny program to do it for me, it's called Kid-Key-Lock. Workes very well with Launchy, and it's free. With a alt+space, k and enter my keyboard and mouse are locked, and with a password sequence of keys that you can choose - in this case the perpetrator's name (that's the cat) - it is unlocked. You can choose to let it display the password while locked if you're just trying to block small children and pets, or if you have extremely bad short term memory.
There is also a shareware version called Pro-Key-Lock where you can customize exactly which keys and combinations of both keyboard and mouse to lock. Probably useful for someone.
3 months ago