I just reinstalled IWD and HOW with the TotL to beat the game, but i notice that when my party is engadeged in combat, the game slowes down and the PC's move slow, very slow.

If anybody knows why this happends or if it is a well known problem, the pleas reply...

Try this:
Try turning off all or most of your feedback options.
Supress ambient sounds.
Reduce your pathfinding nodes.
Increase the size of your cache in the game. (and delete your cache file before you start up again).
And last but not least, try reducing the frame rate.

All of those should help speed up performance.