From etc/PROBLEMS:
When "ClearType" method is selected as the "method to smooth edges of
screen fonts" (in Display Properties, Appearance tab, under
"Effects"), there are various problems related to display of
characters: 2-pixel trace is left behind when moving overlays, bold
fonts can be hard to read, small portions of some characters could
appear chopped, etc. This happens because, under ClearType,
characters are drawn outside their advertised bounding box. Emacs 21
disabled the use of ClearType, whereas Emacs 22 allows it and has some
code to enlarge the width of the bounding box. Apparently, this
display feature needs more changes to get it 100% right. A workaround
is to disable ClearType.
Try disabling ClearType and seeing whether that helps. If so, it's a
known problem, not easy to fix.