Here's the problem: as soon as windows starts up, the fan immediately shuts down. (But for some reason stays on during safe mode.) This is a new fan, the old one was replaced because I thought it was defective. And I'm pretty sure it's not some kind of "smart" fan or motherboard that'll adjust the fan speed.

The relevant specs:
HP 503n (Yeah, I know but it's my office cpu)
Motherboard: trigem 845GL
CPU: 1.7 Celeron GHZ
RAM: 128 mb
Win 98 (XP had a similar problem)

Are you using any fan monitoring software which could be the cause of this (ie. set to silent mode)?

You have connected the fan to the correct fan header (so that it's not mistaken for a different type of fan (ie. system fan instead of cpu))?

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Thanks for the welcome!

As regards to your question, No I am not using any type of fan software. In fact, I just recently reinstalled win 98. It has the bare minimum of software: video & audio drivers, office, and a few misc.

I'm pretty sure it's connected the right header (you mean the power wire, right?) I connected the new fan to where the old one was connected (and the original was untouched from the factory).

it can be your Power supply you might need a biger power supply [/B]

what size is the one you have?

the reason i think this is the computer is not using as much power in safe mode as it do when windows is booted up right.

All you mean is the "max output power", right? (this is what it says on the of the power supply box). In which case it's a 200-watt.

200W is not big you might need to get at less a 350W

are you ruing a DVD ROM or any Cd Buners?

All I'm running is a single cd burner. But I think you're right, the problem may be that it's under-powered or that the power supply is defective or both. I've been running some more tests on the computer: the fan seems to jumpstart when the CPU is under a heavy load (like ripping/encoding a CD or virus scanning). I've asked around elsewhere and they tell me that this is normal for some computers. But it will still power down unpredictably. For example, today I ripped a CD; halway thru, the fan kicked in; after the ripping was finished, I tried to connect to the web, and the system powered down.