What is the difference between a 'custom' install and a 'clean' install?

"There's no real difference between the two, except that the custom install will keep your files where the clean install will erase everything (so you should make a backup first).

IMHO, it's better to do a clean install when switching from an OS version to another. This way, it gives you the opportunity to clean all those superfluous documents/applications you've kept for years and that eats a lot of space, correcting some mistakes you may have done (uninstalled software that are still in the registry, etc...), start from scratch with your favorite tools (up-to-date antivirus, fresh new graphic card drivers, defrags and cleaning tasks, etc...).

It may take some time to backup everything you really need (firefox profiles, game saves, documents, etc...) but once it's done, you start all fresh and clean and your system will obviously be faster."

"A Clean install is usually defined as formatting the driver (either by the user or by using Setup) and then installing the OS.

A custom install is defined by Microsoft as installing the OS on the same partition as a previous installation of Windows. Note that this is not the same as an upgrade.

Let's pretend your harddrive has one partition and contains a previous install of Windows (let's say Vista)

The difference between a custom install and a clean install are the following:

You do not format the partition containing the previous installation of Windows.
Your old Program Files, User Folder, and Windows Folder is moved into a folder called Windows.old
If you didn't have a 100MB System reserve partition before, Custom install method will not create one.

That's pretty much it.
Your old boot files are overwritten. You start out with a clean copy of the Windows folder, Users folder and Program Files folder.

This option has existed for a long long time, but the implementation has changed. A custom install of Windows 3.1 onto a partition with a previous copy of Windows 3.1 will only move the old Windows folder into Windows.old.

This option had also existed under 95 and XP. But like I said earlier, the implementation was slightly different.

So I'd like this to be a PSA and educate people into the fact that you don't have to friggin reformat your harddrive to install a clean-as-a-baby's-behind copy of Windows on your machine.

edit:

For more citation, look here: http://windows.microsoft.com/en-US/windows7/help/upgrading-from-windows-vista-to-windows-7-custom.

Formatting your hard disk during Windows 7 installation isn’t necessary. If you want to format your hard disk, and you’re using an upgrade version of Windows 7, don’t use a program from another software manufacturer to reformat your hard disk prior to installing Windows 7. Instead, start your PC using the Windows 7 upgrade installation disc or a USB flash drive, click Custom (advanced), and then click Drive options (advanced)."