CMOS battery is well... a battery. it looks like this in most motherboards.
this litle thingy power your computer's CMOS which stores your BIOS setup. CMOS is basicly a big IC tats juz a few milimeters smaller than your processor. it usually hv to words 'CMOS' printed on it. BIOS is your computer's basic programming if u would like to call. it stores basic data about your hardware such as where's your cdrom, info bout your hard disk, clocks etc. so if the CMOS battery runs dry, your clocks stop(tats the nice part), then your computer fails to boot(which is really, really nasty).
to change it, all u need to do is to crack open your CPU(nicely of course). n find this battery. its bot the size of a fifty cent, silver in colour n usually near the CMOS. n it is sometimes covered by a black plastic cap.
when u eventually found it, pry it out gently with a screwdriver. but becareful not to touch other stuff. n then check the number on the battery n get a new one n put in in the same way u take it out.
when you're done, turn on your computer. u'll c the black screen with white sentences as u usually do, n look at the left-bottom-corner of the screen. it says "press F* to go into setup". press it. n a blue screen comes out. this is your BIOS setup.
ok, go explore a bit in here n DO NOT change anything unless u're intended to. find a sentence which says "load.......". its usually above the "save n exit" n its different for every computers. u'll use your arrow keys in here instead of your mouse. select it n press enter. a pop up will come out, press "y"n hit enter again. your computer will reboot(restart).
n when u log in to your account. move your mouse to the clock in your taskbar. i know, its still at 12am, but tats normal. connect to the internet n synchronize your time. the time would b back to normal now. if your time is already back, then u hv a really good job of buying motherboards.
after this, everythings is normal again. if you're a bit paranoid, u can always go to the motherboard's manufacturer's website n update BIOS. there's a tutorial there so i ont hv to type so much.
oh, n a reminder. CMOS battery typically last 2 to 10 years. so check regularly if your computer is older than 2 years.
good luck.