Brush problems

I've been having a bit of a problem recently, with the brushes in Photoshop 7. Usually when you use a brush you can see the outline of it while you're placing it on your image - mine however sort of randomly turns into a tiny point, looking much like the 1px round brush, and it's impossible to change it back to normal mode.

Now, I have no idea *why* it does this and no idea how to change it back once it's happened. Sometimes it works to exit the program and restart it but occasionally I have to reboot the computer in order to make the brushes work properly again. As you can imagine, this gets annoying after a while.

Does anyone else have this problem? Does anyone know how to fix it? Help please?

Thanks