How do i stop Quicktime from opening previous videos?
That feature is part of Lion and QuickTimeX, but you can disable it.

Open your terminal (/Application/Utilities/Terminal.app) or use Spotlight to find the Terminal and type:

defaults write com.apple.QuickTimePlayerX NSQuitAlwaysKeepsWindows -bool false


That should stop QuickTime from saving the opened windows when you quit it.

If you want to revert the behavior, change the "false" to "true".