As a kid my dad had a Amstrad PCW with CP/M-80. Loved that thing - kept it until 2008 or so and it kept working, but I had to chuck it (worst decision ever - had the manuals, printer, everything) when my parents moved. Next up was our first PC in 1994, a Cyrix 486 with 4 Mb RAM and Dos 6.2 + Windows for Workgroups. In 1998 came a Gateway 2000 PC running Win95, got the 486 and dug around in the OS, modifying (and breaking :stud) it quite often.
Got my first PC with Win98SE in 2001, crappy machine that failed pretty soon.
At Uni we had Macs running MacOS 8.something, so I got me an eMac in 2003 running MacOS 9.2.2 and MacOS X 10.2 (one of the last dual boot models).
Dad got a new PC in 2004, running XP (me: "Why the F... can't I configure this thing easily?"), so I got the Gateway, upgraded it using parts left from my 2001 PC, loaded Win98SE and it still works very well to this day.
At Uni/work we had XP, my dad used XP (and I did the "support"), and I grew to loathe the system. Unwieldy, hard to figure out what the system was doing and why, the interface, the non-stop issues with virusses, etc. Got myself a PowerBook in 2005, running MacOS X 10.4, generally happy about it until I needed to do more complex tasks involving the command prompt and discovered I couldn't get a newer Perl to run properly because something was wrong and half of the man pages were missing.x( So I looked elsewhere. I remembered reading that OS X was partially based on a BSD, so I had a look at it. Decided some time later that if my next system was to be a PC, it'd run FreeBSD.
Had my first taste of Windows 7 18 months ago on a work laptop, hated it. Had to set Win7 up for my dad on his new PC, hated it more, especially after discovering Microsoft had managed to break compatibility between Outlook Express and the new Windows Mail (on the other hand, got my parents to use Thunderbird for their email and they like it - score one for Open Source). And since I got my dad's old PC again...:e