Not really. With Windows, the program files are necessary, but they are not the entire picture. The reason you have an InstallShield process is so it can make all the necessary changes to the registry - some can number in the dozens or even hundreds of "tweaks", additions, changes, etc. for each program. So, if you have only copied the contents of the system drive to an external, your only option is really to copy them back - but this won't bring back the registry.
To do that, you need to create a system image backup (I can't remember if XP has this option built in or not).