Should program files or system files be backed up?
When you do backups of your hard drive in case you have a failure, you should only back up files you have created. You should not backup system files or program files. You need to use the install program for that program.
There is several reasons why you can't just copy over system or program files.
A program installs numerous files. These files get installed in different locations, these locations can vary depending on the program. They can be installed in their own directory, under program files, and even windows to name a few. Unless you know all the locations, it will be easy for you to overlook a location to install a file.
Several programs may use the same file. For discussion I am going to use Quicken, Outlook and Word as an example of three programs. Each program uses a file called example.dll. When you install the first program Quicken, Quicken installes example.dll to the proper directory and tells the registry to refer to example.dll for version 3 because that is what Quicken states is the version. You then install Outlook and it installs example.dll to its proper directory and tells the registry it is version 4. The installer tells the registry to use version 4 because it is the newer version. Now Quicken and Outlook will look for version 4 of example.dll and it works fine. You then decide to copy the program files of Word to the system instead of installing them. You copy the example.dll from Word to the proper location but it is version 2. You try to use Quicken but it crashes. Gives error messages and other problems. Same thing happens with Outlook. That is because they are trying to look for version 4 but now it is version 2.
Using a install program to install the program is like moving and sending in a change of address card. It tells everyone where you moved to, so they know where to mail you your mail. Doing a copy is like moving without putting in a change of address. No one knows you moved so they send the mail to the wrong location.
It is ok to copy install programs that you have downloaded but not the system files. If you want to create an image to reinstall with, you can use an imaging program such as Ghost from Norton. There is other programs that does it too but I am not familiar with the others. An image is an exact replica of your system. If you have a failure and need to restore your system, you can use the image to do that for you. When you make the image make sure the image is error free, virus and spyware free or your image will contain them.