Possible power supply fan issue w/ Win 7

I haven't kept up with power supply technology at all, on my old PCs that ran Win 95 thru XP, I generally had 250W-450W power supplies that were pretty noisy. The power supply fan ran all the time, and expelled quite a bit of warm air.

On my new PC with Windows 7, I have a "700 watt" power supply that seems to work OK, but I notice that after Windows loads the power supply fan seems to stop running, or runs so slowly that you can't hear it. There is very little air movement coming out. During bootup the power supply fan (I think) whirs and hums like I'd expect; but the instant the Windows7 logon screen comes up, the fan goes silent. Are power supply fans supposed to be variable speed or load-dependent, now? Is the power supply tied in to the OS somehow?

Thanks... although everything seems to be working fine, I don't want to fry my PC and/or power supply.

You didn't specify brand/model of the PS, so I can't make a flat statement, but yes many PSs have internal thermal controllers which only run the fan when needed. I'm not aware of any direct control of this fan via the OS, but when any Windows OS finishes loading they power requirement would lower, thus reducing the need for the fan. My PS has both a bottom fan for intake, and a back fan for exhaust, and only the intake fan is thermally controlled. I might be somewhat more concerned if the exhaust fan wasn't spinning.