I’ve done a bit of programming over the years, and I’m currently working with a Visual Basic 6 (VB6) code base.  I’ve found the following quotation so true:

It is a weird thing though if you’ve noticed that for very old applications, you know, I feel like for some of them you often have multiple ways of doing things, they just end up co-existing.

…

It’s kind of like you look around and you can say, oh, man invented fire here, and it’s like, oh, man discovered wheel here and you’re finding the entire history of 15 years of software development stuck in one codebase…

Michael Feathers, speaking with Scott Hanselman, on Hanselminutes podcast 165.

Which you need to know BEFORE you rebuild your PC.  (So you can take a backup copy of it).

Not that obvious until you need to know where to find it:
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\Outlook\9375CFF0413111d3B88A00104B2A6676