Making changes to critical (and less critical) system files

From time to time, you might want or even need to make changes to system files that are more or less critical. This can be risky business, and as a result, it is always smart to make the changes in a non-operative version of the file, then back up the file, and only then implement the new version.
 
A practical example; You want to change the OEM-info of your computer. The way you do this is first to locate and make a copy of the OEMINFO.ini-file, then make the changes in the copy. Once you’re done, you rename the OEMINFO.ini-file to OEMINFO.ini_old, and only then do you actually implement the new file, copying it back in.
 

 
The reason for this is twofold; firstly you’ll not be screwing up anything while editing, and should something go wrong, it’ll be that much simpler to roll back; simply rename the original file and you’re back up and running.