A friend of mine had a Presario C300 that would not turn on. The power light by the front speaker would just flicker when I plugged in the power cord. So I used the power cord from my Presario V2000 and the C300 still had the same problem. So, I took the C300 apart. Short story, I took everything out except the monitor and mother board, blew the dust out, and put it back together and it worked like a charm.
Long story: I first took out the hard drive then plugged in the power cord and tried turning it back on, the power light would just flicker. Then I took out the RAM, CMOS battery, Wi-Fi card, keyboard, internal power supply cable, CMOS battery, CD drive, heat sink, fan, CPU, and then the speakers. Each time I took out a part, I would plug in the power cord and attempt to turn on the laptop but the power light would just flicker. I blew out the dust with an air can each time I took a piece out since there was so much dust mainly around the fan.
At this point, I figured that the mother board was bad and should just return this busted C300 back to its owner. After I had reassembled the laptop, I was surprised that I had no extra screws lying around. I thought I would try to power it up for fun. Success!
Go to HP’s website for the documentation on taking apart the C300. Tip is to start by putting a flat head screw driver under keys F1, F5, F11, and Insert to pop off the top cover. Unscrew the keyboard, unplug the keyboard, and then flip the laptop over to take out the other components. Remember which order the cables go on the Wi-Fi. Use a marker to note which color cable goes where on the Wi-Fi card. I hope you have the same luck.