i suggest replacing the keyboard-- here are instructions how to remove keyboard
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if you go to this link to make a purchase, http://www.hootoo.com/acer-travelmate-2420-laptop-keyboard-p-51629.html?gclid=CJOHkbernaYCFQO8KgodvQmzoQ
detach the power cord from notebook and remove the battery.
You need to remove the cover where the power button is located, be careful as there is a ribbon cable attatched to the mainboard under here to pulling to vigorously may cause the cable to get pulled out. Then you need to remove the screw(s) holding the keyboard down, once this is done lift the keyboard away from the and lay it face down on the unit so that you can see the mainboard. You need to look where the keyboard connects, to the left of it you are looking for a switch block, if you have one of these then you need to switch the switch labelled "1" to the other position and turn the unit on and enter the bios, this should bypass the password. Once done turn the unit off and and put the switch back to its original position. Then you need to put the unit back together.
If you dont have a block of switches you're looking for two triangle shaped solder contacts on the mainboard in the same area to the left of the keyboard connect, they should be pretty easy to find as they are the only triangular shaped contacts around here, you need to short these out, and turn the unit on while still shorting them and enter the bios, this should clear the bios."