If you have to leave your PC alone for long periods of time then there’s always the worry that someone could access it. Of course, you can always use the Windows keyboard lock to block intruders but this is by no means is the most secure way of locking your PC and can be easily hacked.
A better option is PC Locker Pro which sits quietly in your system tray and can be activated with a hotkey. This blocks both the keyboard and mouse and requires an encrypted password to reactivate it. Slightly more advanced is Locked! which does pretty much the same thing although it also logs unauthorized attempts to access to PC so you’ll be able to see if anyone has been trying to access it. The most simple solution you will find is Lock MY PC which installs into the context menu and can be activated simply by right clicking. The good thing about this program is that it still stays active even if your PC is turned off and on or crashes. Alternatively if you’re only looking for a PC lock that activates after a short amount of time away from your PC, ScreenLock may be your best bet as it locks the PC as soon as the screensaver activates.
Note that it’s a good idea to lock your USB ports if you are particularly worried about your PCs security. Via the USB port is the most likely physical way that someone could plug in an external device and gain access to your PC. In which case, it’s not a bad idea to install USB Lock Auto Protect locks your USB ports with a password and this also gives you peace of mind that no one can steal data from your PC.