Could you suggest some houseplants that are small enough to set and grow on a windowsill?

There are many, many plants small enough to grow on a windowsill. Here are a few suggestions. Cacti and succulents not only grow very slowly, but are very compact and easy to grow. They are also one of the most interesting-looking group of plants. In the flowering plant category, you could try the ever popular African violet or maybe something a little different such as a kalanchoe or miniature cyclamen. If you like trailing plants, try English ivy. Ivy grows quickly, but is easy to prune and keep at a manageable size.

A vining weed climbed all through a couple tall roses bushes of mine before I was aware of it. It will be nearly impossible to untangle the weed from the rose canes and if I try, it will break off many stems. Is it OK to just trace the vines to their roots and pull out the roots, while leaving the vines to die on the rose canes? Will that cause any harm to the roses?

You are correct in cutting the vine at the root system. Just leave the vine on your rose bush until it has dried. Then through the normal pruning of your roses, most of the dead vine will come out. The speed at which the vine grew hints at it being an annual, which would have died out over the winter.

Every so often when I dig up perennials to move or divide them, I notice that some of the finely shredded pine bark that I have used to amend the soil has a white, powdery coating on it. What is it? Is it a problem?

The white powdery coating that you describe is just nature taking its course. The shredded pine bark that you use is just decomposing, and the white is a natural fungus helping the process. This will not cause any problems for you in your garden. In fact, since you have mentioned that you are adding organic matter to your soil and periodically dividing your perennials, that seems to indicate that you have a healthy growing garden.