There are many ways to execute this prank, some more advanced than others. Because I am presuming a lot of sKitties will read this I will use a way that requires you to have access to the remote computer and allow the prankie to have at least some chance.

We will be using remote assistance.

PS: how come you can't delete instructables that have been in competitions? competitions with lucky draw prizes inspire people to make any instructable they can, and although the comps bring out some good work, they also bring out some worthless work.

Step 1: Make an Invitation

In the start menu click on help and support
Then under the heading "ask for assistance", click invite a friend > Invite someone to help you > save invitation as file.
Type anything you want in the name field and set an expiry time.
Because I'm saving to my thumbdrive, I'm not going to set a password, you can if you want to.
Direct remote assistance to either your thumb drive or a "shared folder" on the network and save.

(please note that I have a phobia of red circles, so I hope you can interpret these pictures)

Step 2: Opening Remote Assistance

BEWARE: LOTS OF GOING BACK AND FORTH

Now you want to go to the controller computer and open the invite there
Go back to the other computer and accept the help
Now go to the controller computer and click take control
Go back to the other computer and accept
Minimize
Now go to the controller computer and wait till someone uses the other computer

Optional stealth:
If you want the prank to be a little bit more difficult to work out (for the prankee) You can start up Powermenu on their computer and minimize it to the system tray, this is a great little program that resides in my startup folder.

Step 3: Final

We just used remote assistance to gain control of a computer with a gui
This is probably the easiest way in terms of being built in and easily accessible, not to mention that you can see and manipulate their screen using a GUI. But there are many more cool ways that you can do thing like this and not even have to have access to the remote computer, if you want you can research these:

BO2K - Black orifice allows you to do things to a remote computer _over the internet_ with no physical access to the remote computer

NIRCMD - A very cool command line program which is great for heaps of things, use its "remote" command to do stuff to computers which you have administrative rights to

There are many more so if you are interested, so do a search of the internets and be rewarded with knowledge.

Oh yeah and here is a video of the process