I, too, have an HP Pavilion laptop. The original battery in mine died at around 18 months of age, as well. I've never heard of PC Assistant, but mine did come with something called HP Battery Check. I'm assuming this is the program you attempted to download, and in all honesty I don't think it's worth stressing over. I have received quite a few erroneous results - and quite often - with it, from saying a good battery is bad to saying a bad battery is good and fluctuating according to how far charged the battery is.
If your battery is still good and your system is being stubborn, this should work. So you might consider trying. It's a tip I got from a friend who works in computer repair and owns the same model Pavilion (dv7) that I have. I forgot what he said was the reason why batteries in HP Pavilions are so moody/problematic, but apparently it's very common in his experience.
In point of fact, my battery erroneously claims it's charged when it isn't at least once every two months, and I'm currently on my first replacement battery. My original battery actually proved to be dead, and when it refused to charge the battery light would blink repeatedly for the entire duration that my system was plugged in, regardless of if it was actually on. Anyway, to the attempted fix.
Turn your computer off. Disconnect the power cable from both the wall and the computer. Making sure the computer's lid is closed, turn it upside down and remove the battery. (Just to cover bases since you said your skills are limited, there's a slider near the battery. It might require a bit of force, but just push it toward the only direction it will go. Generally, there's a picture of an unlocked lock that you'll push it toward. The battery will pop up. Just grab the battery, carefully remove it, and set it aside somewhere safe.)
Now, turn the computer back over and open it. Press and hold the power button for about 30 seconds then release. Nothing will happen and I forget the terms for what this does, but it's a necessary step. Close the computer and turn it back over. Place the battery back in. There's only one way it will go, and you usually have to slide it in first then press down. It will click into position.
Plug the power cable back into the computer then the wall. Ideally, the battery charging light will now appear. Turn on the system and monitor the charge. Once it's mostly charged, or if it refuses to charge beyond a certain point, try powering the system with the battery.
If the battery still doesn't work, then it's likely defective. That or our computers suffer the same further problems. However, in my experience, an alternate/new battery will eradicate a persistent problem while intermittent problems are easily solved by the unplug/remove battery/press power steps above.