LogMeIn Free allows you to blank the screen and/or lock the keyboard with a single mouse click.
It offers the fastest and overall easiest experience of any remote control application I have ever tested.
It's extremely fast to connect, secure, and even the free offering is feature-rich (Wake on LAN works perfectly, but only if there is another machine in the office with LMI installed too). It allows you to copy and paste text and images from the clipboard between the remote and local host- but not actual files.
There is a lot of overlap with what Teamviewer offers: on slower internet connections you tend to find LogMeIn manages to offer a better experience than Teamviewer. The advantage of Teamviewer is that the free version can send all audio back to your home PC so you hear sounds originating from the remote PC (not very useful in reality- I tend to mute it) and it also offers a very powerful built-in FTP client so you can move files back and forth once you are remotely controlling a PC.
Despite all that, I rely on LogMeIn primarily for remote support as the actual remote support experience it offers is the best- hands down. For most people, that is the most important factor- not the bells and whistles. LogMeIn sessions are often able to reconnect if there is a connectivity blip, and it feels slightly more responsive.
The free version of Teamviewer automatically disconnects you periodically, without warning- even if you are still actively using it. LogMeIn do not kick free users offline in the middle of long remote control sessions.
Also, every time you close Teamviewer (if you aren't paying for it) it throws up a "thank you for playing fairly" nag screen. LogMeIn starts and stops sessions without any such popups. They don't need to nag people to get them to pay. I was a free user for 3 years, then finally subscribed to LogMeIn Central so I could add an unlimited number of computers to my account for my business (otherwise you are restricted to "only" 10).
Once you're done, just log out again using the Start Menu, then hit "disconnect". By default, I believe LogMeIn will lock your Windows session if you disconnect or lose the connection anyway. You can also do a regular "lock" by clicking the ctrl-alt-del shortcut key (visible in my screenshot as button 4th from the left with 3 grey squares) then choosing "Lock this computer" from the regular Windows menu. It's very user-friendly.