FoxTab FLV Player is a simple tool which allows you to play Flash (FLV) videos which you might have downloaded from YouTube or other video sharing sites.
The program is completely free, but be aware that by default it will also install various browser addons. If you want to avoid this then don't just keep blindly clicking Next during installation: choose the Custom Installation option when it's presented, and clear the checkboxes next to any of these extras which you don't need.
Once installed, though, you'll find FoxTab FLV Player has a clean and simple interface which makes it very easy to use.
So in a click or two you can open a particular clip, then start, pause or stop playback; jump backwards and forwards in the clip by clicking the video progress bar; adjust the volume, or video size (even playing it full-screen). It's all very straightforward, even for PC novices.