I kept using Windows 98SE for the longest time (up until 2005 I think) just because it ran screamingly fast on my Athlon XP 1700+ and I was still using a couple of DOS apps regularly. When more and more applications started complaining about not being 9x compatible I finally caved in, got some more RAM and switched to XP. The stability of the NT kernel was well received and things got powerful enough to run legacy apps on emulation. Never cared for those 'unattended' versions full of useless eye candy which people seemed to like, I rolled my own slipstreamed SP3 CD tweaked for speed. I skipped the whole Vista fiasco, and even though I once got a serious malware infection just by browsing Google Images (and the anti-virus did absolutely nothing) I learned to love XP. Even after all these years it doesn't feel obsolete, it covers most users' computing needs without getting in the way.
In 2010 I built myself a budget Athlon 64 X2 4000+ based rig with 2GB of DDR2 RAM, 400GB SATA HDD and an EVGA 7800GTX (most parts I had lying around as leftovers from repairs and such) to replace my aging but dependable Athlon XP based main PC. Since I jumped on the 64-bit dual-core bandwagon I decided to try this Windows 7 OS I kept hearing good things about. I ran XP and 7 x64 in dual-boot for about a month to evaluate which one to keep, and I ended up choosing Windows 7. It felt just as fast as XP, with a few simple tweaks I could get it just as I liked, Aero Glass looked nice without being tacky or bogging the system down, and all the other improvements were really appreciated, especially the search box on the Start menu I now depend on. Windows 7 works perfectly for me and with current systems lasting longer I'm all set for a pretty good time.
I still run XP on my netbook (Asus Eee PC 1005PE with 2 GB RAM) because even with an SSD Windows 7 feels sluggish on it, and on a couple of older systems such as my mom's Dell Optiplex GX260 (P4 2GHz 1GB RAM 80GB HDD) which does everything she needs (and upgrading the DDR RAM would be kind of expensive now) and also an LGA775 3.2 GHz P4 box I built from spares for more 'legacy' stuff (like serial/parallel hardware interfaces that only work right on 32-bit OS or my eDimensional 3D shutter glasses). Those systems will continue to run XP for the foreseeable future.