Side-by-Side Installs and Virtual Registries

Windows 98 Second Edition provided a feature called side-by-side DLLs. This feature allowed a developer to use the version of DLLs required by a particular program without overwriting system DLLs (those stored in the \Windows\System folder). This feature worked only on Windows 98SE and only if the program developer took advantage of the feature.

Windows 2000 introduced Windows File Protection, which restored system files automatically if they were overwritten by an application when you installed it or ran it. This protected Windows from crashing but didn't do anything about a program that needed a particular system file version to run.

Side-by-side installs enable users to install multiple versions of the same product on the same computer. For example, suppose Microsoft just released a new version of Office. You want to run a trial version of the new Office suite to learn about new features and test drive it, but you don't want to uninstall the current, well-oiled version. However, multiple programs that use a different version of the same DLL sometimes cause complications-"DLL Hell." When programs use the wrong DLL files, they crash and can take the whole OS down with them.

Windows XP Professional provided a way to handle DLL Hell. It was called Fusion. Fusion allowed programs to install whatever system files (DLLs and others) they needed and redirect any files that would replace system files to the program's own folder. When such a program was run, Fusion created a memory-protected virtual machine to run the program with its own DLLs. The end result was that even if two or more programs were running at the same time, using different versions of DLL or other system files that would "break" the system in past versions of Windows, both programs would run properly. No other programs could touch the area of memory granted to each program. Nor could that program or other programs gain access to the area of memory in which the basics of the OS were running.

Windows 7 and Windows Vista contain a different method from the Windows XP solution. Instead of a single Registry, they use a virtual registry that contains multiple DLLs. A virtualized application registry eliminates any conflicts between software. So, if you want to have two versions of Microsoft Office available on the same computer, for example, the virtual registry lets you run them without conflicts.

Virtual registries also have one other important aspect. Because Windows 7 can create and use virtual registries for data, Microsoft applied this concept to users who do not have Administrator privileges. By using virtual registries, nonadministrative guests can use a virtual registry and will not disrupt the main Registry. Usually, only administrators have the rights that allow one to install software that writes to the Registry. But now nonadministrators can install software and other various programs onto the system without causing any harm. Windows 7 will continue to run and display all software installed but will not experience any permanent effect from the data on the virtual registry.

Note Windows Vista introduced virtual folders, which help users get data quickly and easily. This technology has been replaced with libraries in Windows 7.