@gary4gar
Yes it is true ! I know it is not easy to find as we do not spend a lot of time in any ATM. anyway here is how I found that.
Happened 1 yr back I think. I went to UTI Bank ATM, before inserting the card, I saw a strange message - That was very similar to the one we see in our PC, I mean the message window. The message was something about licensing. License expires on so and so date, so do something etc. It was written 'Windows Server' somewhere in the message.
I posted the incident in Thinkdigit forum but even I was not 100% sure it is windows.
Recently I went to the same ATM. Two girls were waiting outside the ATM and some technicians (electrical section) were doing something in the ATM room. The girl was like worried, she was talking to the other girl - I do not know what my Dad will tell if he know about it. Then she asked me whats the procedure if the ATM card get stuck in the machine !
The problem was power was down and the alternative source, I guess some sort of UPS also went down / failed when the girl inserted the card.
After some time the technicians repaired / fixed the UPS and guess what ?
The ATM machine 'booted' showing the familiar - Windows 2000 boot screen !
Also, ATM and core banking machines (servers) are connected to either BSNL line (yes, that is correct !) or to direct linking (some antenna like that of tata sky) to the satellite or so. But those are not the normal lines I think, and not connected to internet, may be intranet for the particular bank / network.
Edit : BSNL lines - may be different in different places, I mean may be some other provide the lines in other areas of India.