Control HTPC with keyboard/mouse from laptop

use x2x.
ssh to the HTPC with X forwarding

ssh -X HTPC

Then install x2x on the htpc:

sudo apt-get install x2x

Allow xhost connections to the htpc:

DISPLAY=:0 xhost +

finally issue the command

x2x -east -to :0

Note that xhost allow all is unsecure, unless you are on a trusted network. It is advisable to ssh tunnel x2x when running over an unsecure network.