I don't believe your PC has some problem.
1) Have you created the detachment in Mlawa after dragging and dropping the first cavalry unit? If there's no stack other than the HQ of 2nd Russian army, dragging the second cavalry unit does not work properly, or even creates a new stack instead of merging the cavalry unit into the cavalry detachment.
Edit: maybe some resolution problem, that makes you rather difficult such drag & drop operations. Why not using the Army Management Window (look at the 3x3 buttons in the bottom frame after having selected the HQ, I recall the button (to open this window) is the top-right one.
Side question: what version are you playing with?
2) I think you are trying to drag and drop the cavalry unit over Tannenber instead of dragging the cavalry detachment. You must move the stack (detachment), because dragging the cavalry unit over Tannenberg is not allowed for the following two reasons:
a) you cannot detach a unit outside the area where it's currently located
b) you cannot detach a unit in an enemy area (that sounds like what you reported in previous post, that made me think you're doing this mistake).