If what you want to do is operate your computer remotely, I think that's possible... provided you're on a network.
I seem to remember some software a la PC Anywhere that allowed you to control one system from another -- wasn't great, but it worked pretty well. Unfortunately, I don't remember what it was called... just remember taking over a friend's computer with it... ;-)
If you're talking about direct connecting the two systems, ie, Monitor out on the tower to some sort of video in on the laptop... I'm pretty sure you're out of luck. Sorry.
Andrew