What's probably happening is when you let go of Ctrl, you miss clicking on the enemy and click on the ground instead, causing the character to run. The weapon should never change without your authority (except for when they break). If you're in Passive mode (not Combat mode) the character will automatically draw his current weapon when you click on an enemy, and this can be a bow.
If I'm up against one enemy, I don't bother trying to aim at them. I move the mouse where I want to run and click. When I want to fire my bow, I hold Ctrl and click without moving the mouse. There's only one baddy on the screen, so it must be the closest and hence gets targetted. It saves racing the mouse back and forth.