I whipped up the following AutoHotkey code to fix a couple minor annoyances when using the Windows calculator. The most useful change is remapping the Num Lock key to the ESC key, making the number pad much more like a traditional 10-key calculator. The other thing it does is ensure that numbers are sent regardless of the Num Lock state.

#IfWinActive ahk_class CalcFrame NumLock::Send {ESC} NumpadIns::Send 0 NumpadEnd::Send 1 NumpadDown::Send 2 NumpadPgDn::Send 3 NumpadLeft::Send 4 NumpadClear::Send 5 NumpadRight::Send 6 NumpadHome::Send 7 NumpadUp::Send 8 NumpadPgUp::Send 9 NumpadDel::Send . #IfWinActive

If you're new to AutoHotkey, Lifehacker's beginner's guide is a good place to start.