I remember the old days, when many people used Norton Ghost. It was started from a bootable floppy disk and then one was able to create or restore system images.
Today I use the built-in Windows Backup Image system. What interests me most is that the image is being created while the system is fully operational and the application doesn't even ask user not to use other applications. Since the backup mechanism has to move ~100Gb from one volume to another, it is obvious, that system files (logs, configuration files etc.) will be changed during this operation, what - in worst case scenario - may possibly lead to unstable system (one file backed up before some important operation, second one - after).
I guess guys at Microsoft are smarter than I, so mostly likely this situation won't occur. But then how the backup tool overcomes the mentioned problem?
(By the way, recent versions of Norton Ghost does the same - backup is performed on live system)