Hi Nicko! Welcome to PhotonLexicon.
Sounds like you're just getting started. Warning! Abandon all hope of ever having extra money at this point! From now on all your free cash will end up getting spent on your hobby...
To control your projector, you need a controller, which is a piece of hardware that will talk to the projector. You also need software that will allow you to connect the controller to a computer. The software allows you to create designs that you want to display on the projector. The software will also allow you to put together a series of frames in order to make a complete show. The software sends these frames as digital data to the controller. The controller coverts the digital data to an analog signal, which is then sent to the scanners in the projector. This is why many people call the controller a DAC.
Digital to
Analog
Converter..
The poplescan software is free, and it will work with several different controllers, or DAC's.
Here is a thread that talks about the controller that it uses (complete with schematics.) Norm's Homemade Laser Show is another controller that would work, though the link to his website seems to be down at the moment. (You might want to send him a PM, he's on the forums here as Legato.)
On the other hand, if you're thinking about buying a commercial package, I strongly suggest that you have a look at the new hobbyist product that Pangolin has released. It's based around the FlashBack 3 controller (which uses the USB port to connect to the computer) and comes with the LA Studio software suite for designing and playing laser shows. It's $500 for the complete package (hardware and software). Several people here are running the package, and the reviews have been overwhelmingly positive. So give it some thought... (Search for "FB3" here in the forums to find more discussions about it.)
Adam