CPU fan races

The instructions say:
"Hold down the F1 key during the startup process and enter BIOS setup to re-enter/reset any custom BIOS settings." I would also try hitting the F1 key multiple times from the very turn on of the computer because the key may have to register at some point after turning on.

Try clearing the CMOS settings as explained on this page, it should clear out any passwords and will set the BIOS settings back to factory settings.

HP and Compaq Desktop PCs Motherboard Specifications P5LP LE Lithium

As for "The CPU fan runs normally on boot, but as soon as the Windows flash screen comes up, the CPU fan slows way down."
This is quite normal. On start-up the temperature of the CPU is not being read, when the flash screen comes up just happens to be the same time that the computer recognizes that the CPU is cold and doesn't need to be cooled.

On the "Then the installation program starts running and the CPU Fan speed starts fluctuating wildly from slow to extremely fast."
This could be normal as when files are read from a DVD the CPU doesn't work hard, but when it unpacks the files or does calculations it does work hard. Or it could be that the thermal paste between the CPU and the heat sink is not correct. Either dry , not making contact, or even too much. If the heat is not being transferred that would also explain the shut-down.

Fans, mainly the CPU fan, have a speed control which slows the fan down. This is usually controlled by the BIOS. The speed is based on temperature of the CPU. If it's cool with little load the CPU fan will run very slowly and you won't be able to hear it. On startup all fans ramp up to 100% until the BIOS takes over and slows the fan down.

It appears you have a faulty CPU fan and it is causing the CPU to overheat and shut down.
I suggest you replace the CPU fan. remove any dust from the heatsink and put new heatsink compound on the CPU/heatsink.

Your fan is following the heat pattern of the CPU. As your computer does more stuff, like runs more applications, it has to work harder to make everything go. This creates heat that needs to be cooled by the fan. The fan usually has 2 settings, low, and high. When the CPU reaches a certain temperature the fan will switch into high gear to help cool it down.

Normally, when the fan gets loud and races, it means that the CPU is getting too hot. The normal fault is that the case and ventilation block on the CPU is full of dust.

Disconnect the tower and take it outside or somewhere where a bit of a mess isn't a problem. Open the tower and look inside. Is it full of dust?

Using a dry soft bristle paint brush, CAREFULLY liberate any dust and fluff, if you have a hoover, brush to the hoover to remove dust.

The CPU is under a big metal blob with a fan on the top, Underneath the fan there are a load of ventilation duct in a metal block. See if you can clean them out, compressed air in a can is a good way. In conjunction with the brush.

Check all the vents and fans are clear of dust, including the PSU.

Once done, try it again, does it still race / lockup / close down.

If so, then you may need to renew the heat dispersal grease which is placed between the CPU and the Heat sink (Vented metal block under the fan).

The best place to find how to do that is look on Youtube for CPU changing / fitting / upgrading.

Indications of laptop overheating problem:

1. The keyboard and the bottom of your laptop are very hot when the laptop is working.
2. The CPU fans are working all the time at maximum rotation speed and operate much louder than before.
3. The laptop suddenly shuts down by itself without warning. When it just started, the laptop was shutting down after 1-2 hours and how it shuts down after 5-10 minutes of operation.
4. The laptop works fine when it runs idle, but shuts down as soon as you start using any memory demanding applications (DVD player, image editing software, video editing software, etc.).

Solution:
If the CPU heatsink is not clogged with dust and lint completely, you can use canned air and just blow it inside the laptop through the openings on the bottom and on the sides. It’s nice as a precaution measure, but it might not work if your laptop already has a problem and the heatsink is completely clogged.

1. Open the laptop case, so you can access the CPU fan and the heatsink. In some cases you can access the heatsink through the latch on the bottom of the laptop. Sometimes (for example Toshiba Satellite A70/A75) you have to open the laptop case all the way down.
2. Carefully disconnect the fan cables on the system board and remove the fan. If the fan makes unusual sound when it spins (grinding sound), I would recommend to replace the fan.
3. Clean the fan and the heatsink with compressed air.
4. I would also recommend removing old thermal grease from the CPU and applying new grease for better heat conductivity.

Sounds like a defective fan, something having to do with the thermal probe, it causes the fan to speed up when cpu gets hot and to slow down when the cpu is cool. I suggest returning it for another one

the fan is supposed to speed up when the processor gets too hot, then slow down after it has cooled a bit. if it is running more than normal, maybe you should take the cover off the pc and clean the dust out of the cpu heatsink.

This is normal

Your fan speeds up and slows down in response to temperature inside the computer. More so the CPU.

I would suggest getting a can of compressed air made for computers

take it outside, take of the side and blow all the gunk out of there, Really go to town around the fan (you might want to wear eye protection)

thanks bigunit12 - as I suspected, no real concern

expect the noise is a pain.

I do think the HS might be marginal and so I shall replace sometime with a higher capacity model, also maybe increase fan flow capacity and add a case hot air extractor fan.

Just to make sure