These systems are usually windows machines - typically "hardened" to various degrees (lock out USB keys etc) and protected by enterprise anti-virus solutions (mcafee, etc)
The windows build is typically a single "golden image" with a known checksum that can be blasted down to machines over wan/lan during the evening.
Source: I used build and deploy the image to many thousands of POS systems at Dixons Store Groups retail chains (UK)
Doesn't DSG use their own custom made EPOS system? Eclipse? Do you have any experience with it and their security policies?
I've had quite a few experiences as a customer at PC World when they've had "till failures" - ironic for a computer store. They often blame head office for overnight updates gone wrong.
Good stuff!