The only way to truly eliminate this would be if your camera was on the same axis as the projector, however if this were the case it would be blocking the light so we'll have to try and work around this.
The simplest solution is to place your camera below the slide projector but up high enough so it is just out of the way of the image.
This will still result in slight amount of keystone distortion and you've got two choices to deal with it. The easiest way is to just crop the image and since the distortion is so slight you'd never notice (this is what I had to do), the other option is to correct it using the perspective or skew transform in Photoshop. I won't get into the specifics of how to do that but here is a link
to an article that goes more in-depth about how to do it.