This comes up from time to time so here is a summary of several ways in which you can use your Tablet PC's display in conjuction with another machine.
A: Same image on both machines
VNC = RealVNC or variants like Ultr@VNC - open source
MaxiVista see site for current pricing (Note: also does scenario B)
Remote control or remote display. Commonly used to control another machine at a distance, but also useful for "cloning" displays. Great for making the desktop a remote display of what you are doing on your Tablet PC.
B: Tablet acts as Extended Desktop
MaxiVista see site for current pricing (Note: also does scenario A)
Extend your Windows desktop across to machines. Basically treats your Tablet PC as a second monitor.
C: Tablet runs software independently
Synergy open source
ShareKMC - shareware
This situation is different as you running your Tablet PC as a separate machine, but using only one keyboard and mouse to control both machines. You get to use the processing power of the Tablet PC and the Inking capability if you pick up the pen and use it.