I first got XP in late 2001. I actually got it off of my friends Dell lol. At the time, OEM licenses would work on more than one computer so both him and I were able to activate. I'm still using the same copy today and it passes Windows Genuine Advantage.
I got it because I heard that it was a lot more reliable than Windows ME, which I had before XP. At first I ran it on 64MB of RAM but when I made a trip to the US, I bought 256MB since it was cheaper there.
Right now, I don't see many advantages to upgrading to Vista so I probably won't do it as well. I don't think Vista will be any more stable than XP and that is most important to me.
My first XP machine had an OEM cd and was built at a comp show in spring 2003. (Athlon 1800+, 256mb DRAM) I got XP because it was already out for a while and friends said it was worth the upgrade with if I had more RAM. I had Win ME at work.
My previous machine to that had 98SE preinstalled and I didn't want to drop cash into it to boost performance to use XP reliably. (IBM Aptiva, AMD K6/475, 128mb SDRAM)
I worked hard to get it to look like the 'classic' interface. (double click icons, 'old' start menu, using the folder tree in explorer, etc.)
Favorite aspect of XP vs. 98? Not having to partition large hard drives, having a 'folder' button on the explorer toolbar, and having search open in explorer instead of as a 'popup'.
Comments can be a good guage of adpotion of Vista.