Probably around '96, 97ish.
I now play OTTD, but to this day still have a CD version of TTO with the Martian Textures and the World Editor (apparently this wasn't around for long) sitting around somewhere...
I still have a Win 95 install disk too, somewhere, actually.
Sucked when it didn't work on XP, then found TTDPatch, worked on one of my earlier installs on my old computer but refused several installs later (let's just say it was a bit of a temperamental computer).
There's only one other game series that has stuck with me anywhere near as long (longer actually, since my second computer had it) - and that would be the legendary SimCity series - and to this day, all four main versions of SimCity are on my HDD.