I would recommend adding the keyboard from the input plugins. you can find it by clicking insert new object, then scroll down to input and there you'll see the keyboard, click on this to add it into your project, then go into your event sheet or create a character control event sheet, It's very useful when adding lots of animation for your sprite. you can use this to assign custom key controls, for movement angle/stepping speed and even configure the animation to be played or frame to be displayed when a key is down. as for acceleration / deceleration, try setting them to low and see how it goes, I'm no expert on construct I'm learning too. I'm currently working on my first platform, solving problems as I go.
Hope this helps <img src="smileys/smiley4.gif" border="0" align="middle" />