I just recently upgraded my sony vaio laptop pcg-934A with windows xp and installed a wireless wlan1500 pc card. Before installing/upgrading, my battery was working fine, up to 2 hours of watching a DVD movie on it. Now, while the battery shows as 100% charged, whenever I disconnect the power cord from the laptop, the battery power instantly shows up as 10% charged within seconds and automatically shuts off. I have to replug the power cord onto the laptop in order to have power. Is it possible that the battery went bad overnight? I have been trying to figure this out for a week now and am nervous about taking the laptop to a repair shop, only to charge me $100 just to tell me to buy another $100 battery.

Actually call afew of the shops in your area including battery vendors and ask if they can test the battery for you. Most will not charge you to do this.
Unfortunately with batteries there's no rhyme or reason to their failure, ive seen them go both ways; gradually over time and just plain dead overnite.

patio.