If you have a Macintosh, it came with iMovie which is a basic and easy to use movie editor.
How to use: http://www.apple.com/support/imovie/
Recent Windows machines came with Windows Movie Maker
Win 7: http://explore.live.com/windows-live-movie-maker?os=other
Vista: http://download.cnet.com/Windows-Movie-Maker-Windows-Vista/3000-13631_4-10729231.html?tag=mncol;3
Win XP: http://download.cnet.com/Windows-Movie-Maker-Windows-XP/3000-13631_4-10165075.html
Open source video editing programs
OpenMovieEditor http://www.openmovieeditor.org/
CineFX http://www.cinefx.org/downloads/
VirtualDub http://www.virtualdub.org/index.html
List http://osafw.blogspot.com/2008/05/open-source-video-editing-tool-in.html
Review of commercial programs
http://video-editing-software-review.toptenreviews.com/
Adobe Premiere Pro: http://www.adobe.com/products/premiere.html
Adobe Premiere Elements: http://www.adobe.com/products/premiereel/
Apple FinalCut Studio: http://www.apple.com/finalcutstudio/
Apple FinalCut Express: http://www.apple.com/finalcutexpress/