I don't recall any significant problems with 98SE; in fact I've still got it on my second computer. I agree with vapilot2004 regarding Millennium. Horrendous OS and no doubt Microsoft learnt a lot from it too (at the end-user's expense). No gripes about XP - it's never let me down in two years.
I wouldn't have a clue how to get around in 3.1 nowadays, which is what I started on. That and Lotus SmartSuite. We've come a long way since then.