About 13% of Windows users worldwide (70 million) were still on 98/ME as of July last year:
iTWire - Windows 98 users on their own says Microsoft
does anyone still use windows 98? - Tech Help Forums - Powered by FusionBB
The numbers have probably gone down since, but I will wager it's still a substantial percentage. Also, I don't know how many are on Windows 2000. That's quite a few users who can't run IE7 for its security benefits. Maybe they're all corporate users but somehow I doubt that...
Vasily