hp compaq 6720s shutting down

[ratcatcher]

hi guys not been around for some time been on a trip anyhow got back home to find sister has been using my laptop so not too bothered what i am pi>>ed about is i now boot u[p laptop and before it boots into windows it turns off now i have always had to use the laptop via mains power as the battery was shot from the start but now no matter what i do it knocks off before booting into windows so open to any suggestions what to try\buy before wasting money cheers guys