Depends on what you mean by "harder to install malware on". Most of these machines are regular Windows, and inherit the default risk associated with such machines. The only difference is that some might have better hardening.
The windows CE and OS/2 are getting rare. those I have tested all had win XP and win 7. (I have tested nearly every major financial institution in my country)
Whenever I come across a green-screen ATM running decent old software I practically leap for joy. Why? I can get my money in 5 seconds. The new ones, that I guess are running Flash on top of MSIE on top of WinXP, are unbelievably slow.
My favorite are the ones where the developer got the resolution of the screen wrong, and both the horizontal & vertical scrollbars are present. The touch screen is emulating a mouse, so you can drag the bars a little ways.
