I have an Asus CUV4X-E motherboard. Mine uses a lithium CR2032 3volt battery (found it in motherboard manual). I got the battery for $2.49 at Best Buy on sale
If yours is this type, mine just popped out and i put new one in. There is a little latch you push to pop it out, and just replace it. I dont know your configuration, but this is first time i've done this type of in-the-case replacement

It wasnt hard at all.
However, I first had to get the system booted to the bios area to write down all my settings. Becuase once you replace the battery it defaults to whatever, and it will need your settings again.
Once you have written down settings (hope they are still there since your battery died ? I caught mine before it died). Then replace battery.
After replacing battery, boot up into bios mode once again (i use Ctrl Alt Esc just as the system starts to boot to get to mine) then replace the settings you wrote down. I only had to change 4 or 5, most were the defaults.
Then fix the time and date settings in system. Mine went back to 1/1/2000

I sent an email that way before i noticed it
I also had to reinstall iomegaware for my zip 250 internal to get the drive to recognize properly. Thats all i had to do
Hope this helps. I just did it this afternoon
jo