Problem with combo box

Hi everyone, a vba newbie here....i been having some problem with my vba code which i think is a subtle bug that i cannot solve. So here goes....
I am programming a code for unit conversion which the user inputs a value into cell "G5". "G5" currently linked to "feet" in the combo box beside it. When the user click and chooses "meters" in the combo box, "G5" value will automatically change from feet to the new value of metres. However, when i use the keyboard to press keydown or keyup within the combo box, the value in "G5" becomes 0! Any idea how to solve it?
Sorry if i could not make myself too clear but i have a attached my file to it. Thanks in advance to any kind soul to help this vba noob.

Ha not really for the sake of programming combo boxes alone. It was meant to be flexible for many other engineering functions calculations. I got the function working. Just that i do not know what makes the subtle bug that when a user presses a keyup or keydown in the combo box, the value of the cell corresponding to the combo box becomes 0. Thanks for the quick reply anyway