I have a 848p-A7 motherboard with a intel pentium 4 processor. I just got everything, set up the cpu fan, and etc. but whenever i try to start up the computer, the onboard speaker keeps going off. It's a long steady beep with a pause every 4 seconds for one second.I have checked all the memory and tried several different sticks of ram in every possible way, but it still screams at me.

You need the motherboard user guide for clock settings etc but first ensure that the socket set (number and distribution of pins) on the new CPU matches that on your board. You should find a user guide on the manufacturers website, follow links for support and downloads.

Here's the page for your CPU on the intel website:-

http://ark.intel.com/products/27468/Intel-Pentium-4-Processor-541-supporting-HT-Technology-%281M-Cache-3_20-GHz-800-MHz-FSB%29

According to this, *********** board needs to have a PLGA775 CPU socket.

Assuming so, its a case of setting the clock jumpers to match the processor speed.

Hi dont know why new RAM dont work, may be faulty. Go to www.crucialmemory.com
Let them scan your computer and they will tell you what memory slots you have and what memory you should be able to use

This motherboard came out about the time when the bad cap's started being put in boards in China. You might want to look a some of the caps.
on the motherboard to see if any of them look bad. A lot of the motherboards I replace have had them . This is one of the symptoms of a bad cap. You might want to check out Bad caps.net for more on this.
I don't know how familar you are with electronics but in case you need to know a cap is the thing on the board that looks like a tin can. If you see any brown stuff comming out of the top or bottom ,or if the top looks puffed up that cap is bad,and you have just found your problem. The motherboard can be repaired or replaced.

Asus P5P800-MX Motherboard Features

64bit CPU support
64-bit computing, the next generation technology to replace current 32-bit architecture, delivers advanced system performance, faster memory access and increased productivity. This motherboard provides excellent compatibility and flexibility by supporting either 64-bit or 32-bit architecture.
Intel EM64T
Intel EM64T (Intel Extended Memory 64 Technology) allows processors to run newly written 64-bit code and to access larger amounts of memory, which delivers exceptional graphic performance and higher data processing speed. Digital media applications can take advantage of Intel EM64T in high quality video and audio processing as well as 3D rendering.
Intel EIST
Intel EIST (Enhanced Intel SpeedStep technology) automatically adjusts Intel 64-bit processor voltage and core frequency according to system performance demand, establishing a self-managed, lower-power consumption, less heat and quieter environment. Combined Intel EIST and ASUS Q-fan technology (adjusts fan speeds according to system loading), this motherboard provides excellently quiet, cool and efficient platform for Intel lovers.
Intel 865GV chipset
Leveraging the Intel 865GV chipset to support P4 CPU on 800MHz FSB, Dual-channel DDR 400 memory, Intel Hyper-Threading Technology, and a full-range of advanced features, P5P800-MX really delivers a full-value, high-performance solution for today's most demanding tasks.

Intel LGA775 Pentium 4 CPU
This motherboard supports the latest Pentium 4 CPU from Intel in LGA775 package. With 800MHz FSB, 1 MB L2 cache, Hyper-Threading Technology and core-speeds up to 3.6GHz and beyond, Intel's LGA775 Pentium 4 is one of the fastest desktop processors to date.

try to update your bios first.

I suggest this is not going to work.
1.Your motherboard is supplying 1.75 DC volts of electricity to the processor socket.
You have installed either a SSpec number SL68Q, or SL62P,
Intel Pentium 4 processor. (The sSpec number is on the top of the processor case)

These Intel Pentium 4 processors use 1.425 to 1.525 DC volts of electricity. If you haven't already, you are going to burn this processor up.

Also, the Intel Pentium 4 that you had before, is based on the Willamette Core. (180nm) These earlier Pentium 4's did not have HT.
Hyperthreading Technology.

The Intel Pentium 4 you have in it now, is based on the Northwood Core. (130nm) These Pentium 4's come with HT.

Even if you could go into your BIOS Setup utility, and change the CPU voltage, and lower it, the HT in the processor you have now, wouldn't work on this motherboard.
(CPU stands for Central Processing Unit. Another name is Microprocessor, or simply -> Processor)

You won't be able to lower the voltage in BIOS Setup, for this processor. IF you did, then you would change the Front Bus Speed, and the ram memory wouldn't work.

Solution?
Obtain an Intel Pentium 4 processor, that uses the Socket 478 processor socket, has a 400MHz FSB, and is based on the Willamette Core.

Here is a list of Willamette Core, Intel Pentium 4 Microprocessors, that will work,
http://en.wikipedia.org/wiki/List_of_Intel_Pentium_4_microprocessors#.22Willamette.22_.28180_nm.29

If by "Reboot" you mean turn-off and restart, your CMOS battery is probably bad and not able to supply enough power to the bios to retain your time settings. Replace the CMOS battery.

Many web sites require the proper date and time on the surfer's PC. For example, Microsoft Update will not allow downloads if the surfer's PC date and time are far out of syncwith the real time. Ironically, even the Update Time function on the surfer's PC will not work if it is too far out of sync.

if i understand correctly you swapped the cpu to another computer and it did not work... you need to purchase or find another cpu that is compatable with your motherboard.

Robert

When you say power is going to the mobo, are
other fans and lights turning on? You also may have forgotten to plug in
the large ATX power cord from the PSU, or the smaller 4-pin rectangular 12V
power cord. Is the correct local voltage setting for your PSU selected (110
or 220)?

I had this exact problem with a newly bought motherboard and it turned on fan turned but the screen was black, also I noticed once it turned on it wouldn't turn off, (Am wondering if this is the case with you) the motherboard was fine as it is in the other room now working away, so dont get rid of yours just yet. Most new motherboards require additional power to the processor from the power supply, through a four pin power cable, which should be availble with the newer power supply units having (number of pins for you motherbpard power supply) + 4 (which is the 4 pin J-connector) printed on the packaging. If this is connected and you are still having problems which for me was the case, then you will have to try a different approach. Cutting to the chase, the problem I had that produced similar effects of screen being black when the motherboard was powered on, was to do with my case. It was one of those flimsy aluminium foil types. I came to this conclusion by taking my motherboard out of the case and hooking it up to the cases power supply on an insulated surface such as a cork mat. It then was working fine. So I bought a good qulaity case with a new power supply just to be sure, hooked everything up and hey presto it was fixed. It turned out that enough power couldn't get to the board as the cheap case was shorting its supply. Hope this is of some help.