r u sure it's a generalized fact or is it just your pc? it's well possible anyway, maybe bcos the idle program is not as good. i know win98 definatley runs hotter than win2000, and i have to use cpuidle, but under 2000 theres no need.
Win95 based OS's don't execute the HLT command when they're doing nothing. I read somewhere that starting from 98SE, they should, with ACPI installed. I really don't think they do, but I could be wrong.
NT based OS's do execute the HLT command.
That's where the temp difference comes from.