If you use CC's registry clean then taking a backup is the default, as is putting the backup in My Documents. If you think that you took a backup then search your c drive for .reg files. If you find one then check the date and time, it'll be called something like cc_20090208_012801.reg.
When you run Registry cleaner then Scan for Issues then Fix selected issues... then click Yes on Do you want to save backup changes to the registry? then choose My Documents and that will save the registry entries with the name like cc_20090202_074754.reg that is cc_today'date_time.reg in that destination Folder.
If you use CC's registry clean then taking a backup is the default, as is putting the backup in My Documents. If you think that you took a backup then search your c drive for .reg files. If you find one then check the date and time, it'll be called something like cc_20090208_012801.reg