Acer AM5641 Desk-top PC

CPU fan is on all the time in computers.

Most any standard PCI NIC will work for you, as long as you have an available port on your PC. Here is one from newegg.com at a great price. It's a Rosewill and I've bought that brand often.
Rosewill RC 404 Ethernet Card 10 100 1000Mbps PCI 1 RJ45

Recover to original state using Acer eRecovery Managemet tool.
press alt+F10 during boot-up process.
This opens eReovery main page.
unless you have changed it, the password is six zeros (0s).
Click recovery actions.
Select RESTORE FACTORY DEFAULT IMAGE.
Once you've confirmed that you wish to proceed the computer will reboot and start to restore the system from the image contained in the HIDDEN PARTITION on the hard drive.

you cpu is getting heat due to which it freezes

Use CPU-z to see which motherboard your system has.

http://www.cpuid.com/cpuz.php

Download driver for your motherboard from Acer

Try installing new memory

If you removed the memory modules and there is no change then I would start disconnecting one device at a time to see if that is causing the problem. Start by disconnecting the cd/dvd rom and then try booting the computer. Then try disconnecting any floppy drives or pci cards(except for video card). If it has a pci/agp video card then you will need to remove it and test it in another computer or test a working one in the problem computer. If it has built-in video then you will need to test a working video card in the problem computer. It could also be an issue with the power supply so remove it and test it in another computer or swap it out with a known working power supply. Also check the mother board screws that hold the board to the chassi are tight(the board uses them as a ground). Blow any dust out of the case and verify that there are no loose connectors. Also verify that the monitor is not the problem by testing it with another computer. Usually when a CPU is defective the computer will not respond to anything and will act "dead" so it does not sound like the CPU. If everything else checks out then it could be a bad motherboard.

PLEASE CHECK THE HARD DISK

Yes definitely. Install any good sound card into a PCI slot. If you have WinXP Pro. as OS then it will automatically pick up the driver. If you are using Vista then buy a Vista compatible sound card.....sodeep

there are two different kind of connectors for drive compoonents such as dvd drives: ide and sata. most computers have both nowadays, though.