Re: What OS & server platform was infected?
I've worked with (not for) a lot of retailers & F&B outlets, and talking to their POS vendors. The 5 POS vendors I've dealt with, which account for most of the worldwide market (4 out of 5 were global vendors) use Windows, with the only differences being the flavour of database (MSSQL, OpenSQL, MySQL, whatever).
This goes for the POS systems in the front-end as well as the back-end.
I even came across one restaurant where all 5 payment positions were using the public wifi, and could be accessed by just going to //[insert terminal position]/ and using default passwords, and found entire partitions as shared.
Anonymous, obviously