I had 98. Now I have XP, with 98 as dual-boot, though I don't want to use that :p . I've had no unusual problems playing QfG1-5 under XP. I do have problems with some things in the KQ collection, but I'm not sure if it's OS-related.

The only thing I really dislike about XP is that you can't take screenshots of DOS games :/ . I might have to reboot to get those.