It sounds like InDiSent may have the best way to do what you want with his mention of synergy as synergy allows you to share your keyboard and mouse on your desktop computer between your mouse and keyboard. You just move the mouse to one edge of the screen and it jumps to the other screen to control it via a client program that runs on the laptop. The server program on the desktop would just hide the cursor on the desktop's screen and send the mouse movements to the client. It also will synchronize the clipboard contents of the computers when you copy things on either computer.
Here is just some extra information on synergy and similar software:
synergy has slow to non-existent development at the moment and as such only works with certain systems under certain conditions.
It should work under Windows XP and GNU/Linux systems without much problem but with systems like Windows Vista and higher it will cause many problems problems and be mostly unusable.
I have seen the program Input redirector at http://www.inputdirector.com/ recommended on a few sites before and it is suppose to support Windows 2000, Windows XP and Windows Vista. I have not tried it myself because I have an unsupported system (Gentoo Linux).
Due to it having more active development, it would be better to try it first if you have Windows 2000 or Higher as it should be more stable.
There is also a program made by Stardock named Multiplicty at http://www.stardock.com/products/multiplicity/ which does the same keyboard and mouse sharing as the above programs.
It has a 15-day trial, full version at $29.95 and one with extra features for $49.95.
The extra features are control of 2-6 extra extra computers (which you get for free using the above programs too) and allows you to "Copy files/folders between machines" which I would guess means when you copy files to the clipboard, it will actually transfer them instead not synchronizing the clipboard when you copy a file to the clipboard (Which synergy doesn't do either. I don't think Input Redirector does either but I'm not sure).
synergy always worked for me until I tried Windows Vista with Gentoo Linux so I can't comment on how stable multiplicity is or how useful it's extra features are anymore then I have.