Portable Windows applications

Most software for Microsoft Windows is not designed to be portable. The Windows registry, the way that .dll libraries are managed, and the structure of the Windows Installer all tend to make application installation a one-way event. Mainstream applications like Adobe Photoshop and Microsoft Word, for example, use the registry intensively, and store state information all over the file system, although software authoring guidelines suggest using the registry for settings and the user's profile (in the "My Documents" or "Documents and Settings" folders) for larger files dependent on a specific installation or the user's habits.

In order to make portable applications, software developers must make their software applications leave the computer they run on completely "clean". This means that the application cannot use the registry, nor store its files anywhere on the machine other than in the application's installation directory. When installed to removable media, a program would need to store settings in an INI file (or similar configuration file) rather than in the registry.

One alternative strategy that exists for achieving application portability within Windows, without requiring application source code changes, is called virtualization. By using virtualization, an application can be "buffered" with DLLs that would intercept all file system and registry calls. This virtualization layer would intercept all non-portable calls, and would direct output to files located in the application's installation directory. This approach would leave the application unchanged, yet portable.

 Portable Macintosh applications

Many programs for the Macintosh OS X have an inherent degree of portability as they are packaged as "drag-install" application bundles, rather than as Installer packages. However, many applications bundles are not truly portable as they store their preferences in files on the local disk where the OS is installed. Macintosh applications which are designed to be portable store their preferences in the drive they are being run from.

Double portability

There is a very restricted category of software that can support a sort of double portability, being both stand alone and cross-platform compatible, able to run on different hardware with little or no modifications, perhaps with minor restrictions. One such software is SymbOS, whose main modules can in their present form be executed on both Amstrad CPC and MSX machines without modification. Only some of its bundled applications are hardware-dependent. To a much lesser extent, Macintosh fat binary applications could be considered as cross-platform, but not always truly portable.