The fan that comes with this is extremely noisy. Regular P4 heat sinks have a different footprint so they do not fit. How would I get a quieter cooler that fits?

Pay attention to the noise rating (Sones level) when you buy a new one. The lower the number, the quieter the fan. Price is not the only factor as it depends on the size, the air flow, extra features (like heat lamps), and design details. Generally, quieter fans cost more than noisy ones, but an expensive fan can be noisy too.

P4 what, what does its lid stated, P/n FULL
only P4HT works here, not P4 older than 2003 sold.

so let me guess, you didnt ask DELL first.
for a list of processors this motherboard supports.
and the other processor (unnamed) works? (2nd shoe drops)
Didnt upgrade the BIOS first, so it can support a larger list
of qualified Dell Processors?
RTC coin cell fresh, not dead,? 2.9vdc minimum.
Dead BIOS is wrong processor, or bad RAM.
assumes good monitor and PSU.
and both fans spin, PSU and CPU.
not running CPU nake,d no heat sink and fan is (super bad choice)

steps not taken.
1: ask dell first. (dah factor 1, we are not dell)
2: tell Dell what P4, it is (it has a full number on top that you forgot to state.)
3: upgrade bios if dell tells you it does this version of P4, and id does not? then back to P4 store for correct P4 only DELL told you can work.
"called the qualified Processor list" only dell has. ok.?
4: install it correctly, with heat sink, thermal grease and fan.

Some gamer or extreme motherboards have this feature, but I doubt ACER does. Try blowing out dust with canned air. This really works for me. I have an older machine, and I live in dusty Vegas so I know what you mean.

The other solution is to look for a bigger and quieter heatsink / fan.

It may be from processor cooling fan fixed over heat sink ( ie large metal rectangular with fins).clean heat sink fins and fans.extreme case fan to be changed by service technicians

This indicates that the fan is faulty and possibly the heat sink is clogged with dust.
A noisy fan could have a worn bearing, I suggest you replace the fan.
Check the CPU fan to see if it spinsfreely, if not replace the fan. If the fan and heat sink is clogged withdust the remove the dust and make sure the vents are clear and dust free. Use acan of compressed air to do this.

see do one thing check the cpu fan whether it is working properly and is not producing any noise if it is then probably you need to replace your cpu fan with a new one also makke sure that your power supply fan and chasis fan is working as excess heat gets generated in the system they remove it
to change the cpu fan 1.see it will depend on what kind of cpu socket you have pin or pinless for pin-open the plastic clamp on the fan for pinless -unscrew the heatsink
2.remove the heatsink and replace it it will be much better if you replace the full heat sink with a new one .also don't forget to put thermal grease compound for effivient heat transfer in between the cpu and the heat sink contact generally these days heat sinks come with pre-applied thermal componds but still it is recomended to apply it firmly over the cpu before installing the heat sink on it
3.if you wish to install the old heatsink by just replacing the top fan then replace the fan and rub the bottom of the heat sink with a sand paper to make it even and free from dust and oxide apply thermal compond and install it on the processor .plug bak the cpu fan header cable . start your system it will stop overheating..

As for the green shroud, I would not recommend removing it. The computer case is designed to correctly circulate the air through the system and safely out. As for replacing the fans you can replace them with anything you like, just make sure the new fans are the same dimensions they come in szies 40, 60, 80, 92, 120, 140 and a newer 200 mm form factor so you can just measure them if you don't know the correct size. Next look at where the fans plug in, if it is a three pin plug then they fan's are kept at optimal speeds via the motherboard and bios. So they are already controlled by heat sensors / thermostat so to speak. If they are 2 pin then they just run full speed usually. For the back fan just make sure it is the right size and pin out. You can hook a 3 pin fan to a 2 pin fan plug on the board it will just run full speed all the time. For the CPU fan buy a combo fan/ sink and just make sure the sink is the same as the existing one so it will fit on the motherboard. You could probably get rid of the green shroud if you went with a really high end fan / sink for the CPU but that's up to you. For the thermal paste it is easiest to put a small blob in the center of the CPU and then use a plastic card, such as a drivers license to spread it out to cover the entire CPU, thinly and evenly. Add small amounts if you need to to cover the entire surface. Try not to have a bunch oozing out the sides although it won't really hurt if it is.

That's because it heats up more easily and faster, which usually makes the fans spin faster and results in being noisy. Have you ever cleaned out the interior of the computer? If not, or if it's been awhile since it's been done, then buy some canned air for dusting electronics, open the side of the computer case (while it's not powered ON), and blow out the fans and the heat sink. If you get alot of just clogging up the heat sink, then it will not dissipate heat correctly; which results in the fans running more often. It that doesn't help any, then I would try to figure out if one of the fans is making alot of noise because it's going bad. You can find replacement fans on ebay, or at local electronics\computer shops.

you can contact online for techsupport at dell at www.support.dell.com this a live tech support session totally free they have helped me out numerous times keep me posted on how this goes ok best regards mike

Which one of these CPU's are you using?