The question was about Linux, but as long as we're reminiscing about first installs I'll add that the first time I installed a BSD was sometime around 2003 or 2004. FreeBSD, which I'd been aware of ever since reading the first announcement of it in Dr. Dobb's Journal but which I'd never installed on one of my own machines until then. I had an iBook G4 with the first edition of OS X and had been enjoying its BSD underpinnings via the commandline, so it was inevitable that I'd put a BSD on my Intel box.

FreeBSD was my preferred OS for several years after that, until I switched to Linux to be able to use some programs which were only available there. So while I'd first installed Linux back in the 1990's, I can't say it ever became my everyday system until much later. Mostly it was one of those things where, as it was for a lot of people it seems, the fun was all in "getting it working" -- but as soon as I'd achieved that, I usually set it aside until the next interesting distro CD came along.