- The operating system (typically Windows)
- Installed applications
- User created data (documents, pictures, emails etc.)
When you install a typical application, additional to copying the software to c:\program files, the install process can also modify the windows information store (registry), extend Windows (add background services and drivers) and carry out other actions specific dependent on your computer type and Windows version. This customization and modification of shared resource during installation makes the installed files non-portable between computers.
To further complicate matters, despite there being some standard places for applications to store their data, they are not observed by many applications. Hence, it is impossible to automate the backup / restore of all your user data.