Is it possible to get information about the hardware from an offline version of HKLM/SOFTWARE hive? There is OEMInformation in there, which gives things like machine manufacturer, but does the machine need to be on to get this information?
Yes and no. People can get information about your hardware by opening up the computer. However your asking more specifically about the Registry. Well the machine needs to be on. But you don't need to be booted into Windows. You can access the registry using a Live CD. Using a Live CD on the local computer you can achieve an also lot. Such as run better virus scans, crack passwords, add files to the system, remove files from the system, modify files on the system and much more. However the computer will most likely need to be on and have access to the internet or the network in order for someone to access the registry.