That's really odd -- we don't notice this on any of our Vista machines here.
On a little more research, the DLL which contains the month calendar control that we use is called COMCTL32.DLL -- for some reason, perhaps yours isn't complete on your PC, or an outdated version. You can get an updated one from this zip file -- it belongs in your c:\windows\system32 directory. (Note, these DLLs were taken from a fresh Vista Home Premium machine; do not try to install on XP, NT, 2000, 98, ME, or 95.)