Just a thought, but try these few things:
Try a few options under the "Adjust how PuTTY displays line drawing characters."
(Font has XWindows encoding, Use font in both ANSI and OEM modes, et cetera.)
You can also try changing a few other things in there. Also, make sure you are using an updated version of PuTTY.
I'll look into the problem, and see if I can find anything else on it. If I do, I'll let you know.