Any prior comment you see that may indicate that the described symptoms do not indicate a possible need to replace the CMOS battery are WRONG! Of course you would first try to boot from your Windows OS disk to attempt to repair booting errors due to HD corruption and also verify RAM is ok. Tools for this are on your OS disk if you're using Win 7.
But if you find the PC won't boot even to access the CD/DVD drive and it keeps cycling off after manual attempts to restart the CMOS battery is a likely culprit. You must open the PC to replace that battery.
You may find other references on the web (as I did) that describe how to access and replace the CMOS battery on the ACER EXTENSA 5420-5687. Those instructions are incomplete and partly incorrect. So here I correctly describe what is necessary.
HAVE THE PC TURNED COMPLETLY OFF. REMOVE THE BATTERY AND POWER TO THE PC BEFORE DOING THE STEPS BELOW.
1) Open the laptop screen completely and Locate the rectangular plastic strip along the very top of the keyboard. Remove this strip by gently prying up at either the right or left edge to start lifting it. Then slowly and gently continue lifting it using your fingers. Note that part of this strip contains a contoured portion that covers part of the screen hinge. Also, be careful when lifting this plastic cover not to try to completely separate it from the PC. There are folded thin electrical connections underneath the cover that must remain intact. You just need to carefully position this rectangular cover aside so that you can access and remove the two end screws that hold the display hinges to the bottom of the case. Do NOT remove these hinge screws yet, for reasons explained below.
The goal is to remove and replace the CMOS battery. It's actual position is underneath the plastic bezel that covers the entire remaining part of the laptop once the rectangular strip in step (1) is loosened. The CMOS battery is only accesdible from the keyboard side with the lid opened. Its position is about 1" to the left of the mousepad left edge and about 2" from the front edge of the bottom frame. Because of it's position it is necessary to LOOSEN THE ENTIRE PLASTIC BEZEL that remains so that it can be gently lifted FROM THE FRONT of the frame to access the battery. That plastic bezel is secured by a number of screws; some accessible from the keyboard side, but most are accessible only from the bottom.
To avoid damaging delicate connections between the motherboard and display, we will remove all the accessible screws at the bottom prior to separating the display hinges from the case.
2) Close the lid of the laptop, turn it upside down, and remove the RAM access panel by loosening the 7 small Phillips head screws. Usually these screws stay attached to the access panel as you gently lift and remove it completely. If one or more screws come completely out, be sure to keep track of which hole it went into and safely store the screw for reassembly. All these panel screws are the same size. But there are three other screw types you will be dealing with so keep track and keep them separated.
3) Still with the PC bottom facing you and the lid closed, we will now remove ALL of the screws that secure the top bezel and that are accessible from this position.
There are 12 screws placed along the perimeter of the case. They are all recessed somewhat into their access holes. All of them are the same size. Keep them together separate from other screws of different size.
4) Now LOOK CAREFULLY for any additional similar recessed screws that you can find in the section visible now with the RAM panel removed. YOU MUST REMOVE THE HARD DRIVE because there's at least one or two screws that are not visible until it's removed. The hard drive simply slides out from it's connector once you remove it's mounting screws (keep them separate). In addition to the 12 screws along the perimeter you will be finding up to 4 or 5 more within the rest of this interior footprint that are only accessible from the bottom. Get them all removed now because it will be more difficult if you've omitted one after we take the next step.
It is NOT necessary to remove the keyboard from the large plastic bezel. You may remove it if you choose (as some other guides suggest). But like the small rectangular cover, the keyboard also has a delicate folded cable connected to the motherboard. So you will not be entirely removing the keyboard anyway. I suggest you leave it attached to the large bezel.
5) Now we want to remove all the screws that retain the large plastic bezel and that are accessible from the keyboard side with the lid opened. We must lift this large bezel sufficiently (tilting the bezel up) FROM THE FRONT just enough to gain access to the CMOS battery. There are far fewer screws to remove from the keyboard side. We will start with the two screws at opposite ends that hold the lid