Regshot is a free utility for Windows that allows you to capture a snapshot of the registry and compare it with another snapshot taken after any change has been made to it. The changes between the first and last snapshot are recorded in a text or HTML format file. A free plug-in from Paraglider allows you to save the snapshots in a .reg format which can be easily imported into the registry by simply double clicking on the file. Another add-on called UndoReg analyzes the report generated and deletes referenced registry keys.

This software is useful if, for example, you are planning to install a trial software and would like to be able to revert changes after it expires.

Download Link
UndoReg download link
Paraglider plug-in download link