Hyper-V introduces the concept of virtual machine snapshots, which can be created in the Hyper-V Manager (by selecting a virtual machine and selecting Snapshot from the Action menu or panel) and in the Virtual Machine Connection window (by clicking on the snapshot button in the toolbar or by opening the Action menu and selecting Snapshot...).
Selecting to create a snapshot from the Virtual Machine Connection window will result in a dialog that allows you to enter a custom name for the snapshot (e.g. "Installed Office 2007" or "Just before I do something stupid and ruin the virtual machine"). If you want you can dismiss this dialog and have the snapshot use an auto-generated name. This auto-generated name will consist of the name of the virtual machine followed by the date and time when the snapshot was taken.