Vista Shuts down when Unplugged

I used Bootcamp to install Vista on my Mac Book Pro 2.4 Ghz (April '08). It has been working very well until Dec 2008. While running a PC game on battery, Vista operating system would just shut down unexpectedly. If the machine is plugged in, it would work fine. Currently the problem is getting worse; when I boot up into Vista on battery, it would shut down in about 2-3 minutes.

Initially I thought it was a battery problem. I booted up into Leopard and checked the battery information in System Profiler. I didn't see any problem. 332 cycles and condition is "Good." In addition, Leopard works just fine on battery. Does anyone have any idea about this?

Thank you for all your help!

Do you mean the "Power Options" in Control Panel? There's only three options: "Balanced", "Power Saver", and "High Performance." It's currently selected in "Balanced" and the only settings I can change are "turn off the display" and "put the computer to sleep" for on battery and on AC power. There is also "change advanced power settings" option but I didn't change anything since I installed vista. Thanks.