I did the same thing, but mostly because my computer worked, did what I needed it to do, and I was too lazy to replace it until I was basically forced to.
After building a new PC and switching over to Linux I was like "why didn't I do this a long time ago?"
Internet Explorer was originally based upon the Mosaic browser. Like a lot of Microsoft's tech, it's something they acquired.
Up to IE4, it was a standalone browser. It's IE4 where Microsoft integrated it into the OS and made do double duty as Windows Explorer, which is what you're thinking of.