From Tom Ehlert - Software & Consulting:
Drive Snapshot creates an exact Disk Image of your system into a file, including the operating system, installed programs, your data and all security attributes - while windows is running and you continue to work. If desaster strikes (a virus, trojan, or hard disk crash), a complete and exact restauration of your system will be possible in only few minutes. The image file can be mounted as a virtual drive, so you can easily restore (or use) single files or directories, using the Windows Explorer, or other program (e.g. WINDIFF).
Restart to DOS is only necessary, if you must restore your system partition. Otherwise there is no need to reboot - not even during installation. Drive Snapshot saves only the use disk space, compresses the data and is extremly fast - a P4/2GHz will save approx. 1GByte/Minute on a local disk drive.