In Corel Photo Paint 9, anti aliasing keeps occurring? How to disable it?

For example:

If I have two rectangles next to each other, one is white, one is light grey. If I click the Paint/Fill tool on the grey square, to paint it dark grey, it does so: but I get a medium grey stripe between teh dark grey and the white. It blends / anti aliases, between the two shapes when I fill it in with colour. I have tried setting it so it is color tolerance of zero, and I also changed the HSB tolerance to zero, but it still alters the pixels surrounding the spot I am trying to fill in. In more recent versions of corel, you can toggle Anti Aliasing off, but there's no icon on the tool bar to turn it off that I can see.

So, how do I disable the anti aliasing effect, so that if I click a shape, only that shape colours in, and no blending takes place around it?

I should also mention, I'm on a legacy machine, running Win XP SP 3