Photoshop 6: can't define brush

I've had Photoshop 6 on this computer for about a year now with no problems, and today when I wanted to define a brush (I know how to define a brush, make a brush set, etc, no problems there) the 'define brush' option was not selectable. Does anyone know what might cause this? Seems like everything else works just fine, I used the program yesterday with no problems, but it has been a while since I defined a brush. I cleaned out some of the rarely used brushes I have on there, and I do know that my computer is low on space...but cleaning out the brushes didn't seem to do anything and I can't burn a cd for a while yet. Any ideas? It is a burned copy of PS 6 but like I said there hasn't been anything weird for over a year. I think I can get the copy to reinstall but I'd like to avoid it if I can.

Thanks.